AMENDED AND RESTATED GUARANTY

EXHIBIT 10.71

AMENDED AND RESTATED GUARANTY

This AMENDED AND RESTATED GUARANTY (“ Guaranty ”) is executed as of December 29, 2006, by PRIME GROUP REALTY, L.P. , a Delaware limited partnership (whether one or more collectively referred to as “ Guarantor ”), for the benefit of WELLS FARGO BANK, N.A. , as trustee for the registered holders of COBALT CMBS COMMERCIAL MORTGAGE TRUST 2006-C1 , COMMERCIAL MORTGAGE PASS-THROUGH CERTIFICATES, SERIES 2006-C1 (“ Lender ”).

W I T N E S S E T H :

WHEREAS, pursuant to that certain Amended and Restated Promissory Note, dated of even date herewith, executed by CONTINENTAL TOWERS, L.L.C. , a Delaware limited liability company (“ Borrower ”), and payable to the order of Lender in the original principal amount of $73,600,000.00 (together with all renewals, modifications, increases and extensions thereof, the “ Note ”), Borrower has become indebted, and may from time to time be further indebted, to Lender with respect to a loan (“ Loan ”) which is secured by the lien and security interest of an Amended and Restated Mortgage, Security Agreement and Fixture Financing Statement, of even date herewith (the “ Mortgage ”), and further evidenced, secured or governed by other instruments and documents executed in connection with the Loan, including an Amended and Restated Environmental and Hazardous Substance Indemnification Agreement (the “ Environmental and Hazardous Substance Indemnification Agreement ”) (the Note, Mortgage, the Environmental and Hazardous Substance Indemnification Agreement, and all other such documents and instruments executed in connection with the Loan may be referred to as the “ Loan Documents ”); and

WHEREAS, Lender is not willing to make the Loan, or otherwise extend credit, to Borrower unless Guarantor unconditionally guarantees payment and performance to Lender of the Guaranteed Obligations (as herein defined);

WHEREAS, PGRT Equity, L.L.C., a wholly owned subsidiary of Guarantor, and Lender are parties to that certain Amended and Restated Subordination and Standstill Agreement dated as of the date hereof; and

WHEREAS, Guarantor as the parent of Second Lender will directly benefit from Lender’s making the Loan to Borrower.

NOW, THEREFORE, as an inducement to Lender to make the Loan to Borrower, and to extend such additional credit as Lender may from time to time agree to extend under the Loan Documents, and for other good and valuable consideration, the receipt and legal sufficiency of which are hereby acknowledged, the parties do hereby agree as follows:

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ARTICLE I

NATURE AND SCOPE OF GUARANTY

1.1            Guaranty of Obligation . Guarantor hereby irrevocably and unconditionally guarantees to Lender and its successors and assigns the payment and performance of the Guaranteed Obligations as and when the same shall be due and payable, whether by lapse of time, by acceleration of maturity or otherwise. Guarantor hereby irrevocably and unconditionally covenants and agrees that it is liable for the Guaranteed Obligations as a primary obligor.

1.2            Definition of Guaranteed Obligations . As used herein, the term “ Guaranteed Obligations ” means the obligations or liabilities of Borrower to Lender for any loss, damage, cost, expense, liability, claim or other obligation incurred by Lender (including attorneys’ fees and costs reasonably incurred) arising out of or in connection with the following:

(a)            all amounts for which Borrower is liable under Sections 2.04(b) and (c) of the Note; and

(b)            the payment and performance of all of the obligations of Borrower under the Environmental and Hazardous Substance Indemnification Agreement.

Notwithstanding anything to the contrary in any of the Loan Documents, Lender shall not be deemed to have waived any right which Lender may have under Section 506(a), 506(b), 1111(b) or any other provisions of the U.S. Bankruptcy Code to file a claim for the full amount of the Debt secured by the Mortgage or to require that all collateral shall continue to secure all of the Debt owing to Lender in accordance with the Loan Documents.

1.3            Nature of Guaranty . This Guaranty is an irrevocable, absolute, continuing guaranty of payment and performance and not a guaranty of collection. This Guaranty may not be revoked by Guarantor and shall continue to be effective with respect to any Guaranteed Obligations arising or created after any attempted revocation by Guarantor and after (if Guarantor is a natural person) Guarantor’s death (in which event this Guaranty shall be binding upon Guarantor’s estate and Guarantor’s legal representatives and heirs). The fact that at any time or from time to time the Guaranteed Obligations may be increased or reduced shall not release or discharge the obligation of Guarantor to Lender with respect to the Guaranteed Obligations. This Guaranty may be enforced by Lender and any subsequent holder of the Note and shall not be discharged by the assignment or negotiation of all or part of the Note.

1.4            Guaranteed Obligations Not Reduced by Offset . The Guaranteed Obligations and the liabilities and obligations of Guarantor to Lender hereunder, shall not be reduced, discharged or released because or by reason of any existing or future offset, claim or defense of Borrower, or any other party, against Lender or against payment of the Guaranteed Obligations, whether such offset, claim or defense arises in connection with the Guaranteed Obligations (or the transactions creating the Guaranteed Obligations) or otherwise.

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1.5            Payment By Guarantor . If all or any part of the Guaranteed Obligations shall not be punctually paid when due, whether at demand, maturity, acceleration or otherwise, Guarantor shall, immediately upon demand by Lender, and without presentment, protest, notice of protest, notice of non-payment, notice of intention to accelerate the maturity, notice of acceleration of the maturity, or any other notice whatsoever, pay in lawful money of the United States of America, the amount due on the Guaranteed Obligations to Lender at Lender’s address as set forth herein. Such demand(s) may be made at any time coincident with or after the time for payment of all or part of the Guaranteed Obligations, and may be made from time to time with respect to the same or different items of Guaranteed Obligations. Such demand shall be deemed made, given and received in accordance with the notice provisions hereof.

1.6            No Duty To Pursue Others . It shall not be necessary for Lender (and Guarantor hereby waives any rights which Guarantor may have to require Lender), in order to enforce the obligations of Guarantor hereunder, first to (i) institute suit or exhaust its remedies against Borrower or others liable on the Loan or the Guaranteed Obligations or any other person, (ii) enforce Lender’s rights against any collateral which shall ever have been given to secure the Loan, (iii) enforce Lender’s rights against any other guarantors of the Guaranteed Obligations, (iv) join Borrower or any others liable on the Guaranteed Obligations in any action seeking to enforce this Guaranty, (v) exhaust any remedies available to Lender against any collateral which shall ever have been given to secure the Loan, or (vi) resort to any other means of obtaining payment of the Guaranteed Obligations. Lender shall not be required to mitigate damages or take any other action to reduce, collect or enforce the Guaranteed Obligations.

1.7           Waivers . Guarantor agrees to the provisions of the Loan Documents, and hereby waives notice of (i) any loans or advances made by Lender to Borrower, (ii) acceptance of this Guaranty, (iii) any amendment or extension of the Note, the Mortgage or of any other Loan Documents, (iv) the execution and delivery by Borrower and Lender of any other loan or credit agreement or of Borrower’s execution and delivery of any promissory notes or other documents arising under the Loan Documents or in connection with the Trust Property (as defined in the Mortgage), (v) the occurrence of any breach by Borrower or an Event of Default, (vi) Lender’s transfer or disposition of the Guaranteed Obligations, or any part thereof, (vii) sale or foreclosure (or posting or advertising for sale or foreclosure) of any collateral for the Guaranteed Obligations, (viii) protest, proof of non-payment or default by Borrower, or (ix) any other action at any time taken or omitted by Lender, and, generally, all demands and notices of every kind in connection with this Guaranty, the Loan Documents, any documents or agreements evidencing, securing or relating to any of the Guaranteed Obligations and the obligations hereby guaranteed.

1.8            Payment of Expenses . In the event that Guarantor should breach or fail to timely perform any provisions of this Guaranty, Guarantor shall, immediately upon demand by Lender, pay Lender all reasonable, out-of-pocket costs and expenses (including court costs and reasonable attorneys’ fees) incurred by Lender in the enforcement hereof or the preservation of Lender’s rights hereunder. The covenant contained in this Section shall survive the payment and performance of the Guaranteed Obligations.

1.9            Effect of Bankruptcy . In the event that, pursuant to any insolvency, bankruptcy, reorganization, receivership or other debtor relief law, or any judgment, order or decision

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thereunder, Lender must rescind or restore any payment, or any part thereof, received by Lender in satisfaction of the Guaranteed Obligations, as set forth herein, any prior release or discharge from the terms of this Guaranty given to Guarantor by Lender shall be without effect, and this Guaranty shall remain in full force and effect. It is the intention of Borrower and Guarantor that Guarantor’s obligations hereunder shall not be discharged except by Guarantor’s performance of such obligations and then only to the extent of such performance.

1.10         Waiver of Subrogation, Reimbursement and Contribution . Notwithstanding anything to the contrary contained in this Guaranty, Guarantor hereby unconditionally and irrevocably waives, releases and abrogates until the end of the Waiver Period (as hereunder defined), any and all rights it may now or hereafter have under any agreement, at law or in equity (including, without limitation, any law subrogating Guarantor to the rights of Lender), to assert any claim against or seek contribution, indemnification or any other form of reimbursement from Borrower or any other party liable for payment of any or all of the Guaranteed Obligations for any payment made by Guarantor under or in connection with this Guaranty or otherwise. As used herein, “ Waiver Period ” shall mean ninety-one (91) days from the payment in full of the indebtedness secured by the Mortgage and Borrower’s failure within such ninety-one (91) day period to be subject to a bankruptcy action or otherwise in violation of Section 55 of the Mortgage.

1.11         Borrower . The term “ Borrower ” as used herein shall include any new or successor corporation, association, partnership (general or limited), joint venture, trust or other individual or organization formed as a result of any merger, reorganization, sale, transfer, devise, gift or bequest of Borrower or any interest in Borrower.

1.12         Indemnity . Guarantor shall indemnify, defend and hold harmless Lender and its successors and assigns from and against the following (which may be referred to herein as “ Indemnified Matters ”): any and all liabilities, obligations, claims, demands, damages, judgments, costs and expenses (including, without limitation, reasonable attorneys’ fees and disbursements), imposed upon or incurred by or asserted against Lender and/or its successors and assigns by reason of a default by Guarantor in the performance of any of the Guaranteed Obligations. The foregoing indemnity shall specifically not include any such costs to the extent incurred as the direct result of Lender’s gross negligence or willful misconduct.

(a)            If any action shall be brought against Lender based upon any of the matters for which Lender is indemnified hereunder, Lender shall notify Guarantor in writing thereof and Guarantor shall promptly assume the defense thereof, including, without limitation, the employment of counsel acceptable to Lender; provided, however, that any failure of Lender to notify Guarantor of such matter shall not impair or reduce the obligations of Guarantor hereunder. Lender shall have the right, at the expense of Guarantor (which expense shall be included in the Indemnified Matters), to employ separate counsel in any such action and to participate in the defense thereof. In the event Guarantor shall fail to discharge or undertake to defend Lender against any claim, loss or liability for which Lender is indemnified hereunder, Lender may, at its sole option and

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election, defend or settle such claim, loss or liability. The liability of Guarantor to Lender hereunder shall be conclusively established by such settlement, provided such settlement is made in good faith, the amount of such liability to include both the settlement consideration and the actual, out-of-pocket costs and expenses, including, without limitation, reasonable attorneys’ fees and disbursements, incurred by Lender in effecting such settlement. In such event, such settlement consideration, actual, out-of-pocket costs and expenses shall be included in the Indemnified Matters and Guarantor shall pay the same as hereinafter provided. Lender’s good faith in any such settlement shall be conclusively established if the settlement is made on the advice of independent legal counsel for Lender.

(b)            Guarantor shall not, without the prior written consent of Lender: (i) settle or compromise any action, suit, proceeding or claim or consent to the entry of any judgment that does not include as an unconditional term thereof the delivery by the claimant or plaintiff to Lender of a full and complete written release of Lender (in form, scope and substance satisfactory to Lender in its sole discretion) from all liability in respect of such action, suit, proceeding or claim and a dismissal with prejudice of such action, suit, proceeding or claim; or (ii) settle or compromise any action, suit, proceeding or claim in any manner that may adversely affect Lender or obligate Lender to pay any sum or perform any obligation as determined by Lender in its sole discretion.

(c)            All amounts owed by Guarantor to Lender that are Indemnified Matters shall be immediately reimbursable to Lender when and as such payment obligations are incurred by Lender and, in the event of any litigation, claim or other proceeding, without any requirement of waiting for the ultimate outcome of such litigation, claim or other proceeding, and Guarantor shall pay to Lender any and all amounts owed by Guarantor to Lender that are Indemnified Matters within ten (10) business days after written notice from Lender itemizing the amounts thereof incurred to the date of such notice. In addition to any other remedy available for the failure of Guarantor to periodically pay such amounts owed by Guarantor to Lender that are Indemnified Matters, such amounts, if not paid within said ten-day period, shall bear interest at the Default Interest Rate (as defined in the Note).

ARTICLE II

EVENTS AND CIRCUMSTANCES NOT REDUCING

OR DISCHARGING GUARANTOR’S OBLIGATIONS

Guarantor hereby consents and agrees to each of the following, and agrees that Guarantor’s obligations under this Guaranty shall not be released, diminished, impaired, reduced or adversely affected by any of the following, and waives any common law, equitable, statutory or other rights (including without limitation rights to notice) which Guarantor might otherwise have as a result of or in connection with any of the following:

2.1            Modifications . Any renewal, extension, increase, modification, alteration or rearrangement of all or any part of the Guaranteed Obligations, the Note, the Mortgage, the other

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Loan Documents, or any other document, instrument, contract or understanding between Borrower and Lender, or any other parties, pertaining to the Guaranteed Obligations or any failure of Lender to notify Guarantor of any such action.

2.2            Adjustment . Any adjustment, indulgence, forbearance or compromise that might be granted or given by Lender to Borrower or any Guarantor.

2.3            Condition of Borrower or Guarantor . The insolvency, bankruptcy, arrangement, adjustment, composition, liquidation, disability, dissolution or lack of power of Borrower, Guarantor or any other party at any time liable for the payment of all or part of the Guaranteed Obligations; or any dissolution of Borrower or Guarantor, or any sale, lease or transfer of any or all of the assets of Borrower or Guarantor, or any changes in the shareholders, partners or members of Borrower or Guarantor; or any reorganization of Borrower or Guarantor.

2.4            Invalidity of Guaranteed Obligations . The invalidity, illegality or unenforceability of all or any part of the Guaranteed Obligations, or any document or agreement executed in connection with the Guaranteed Obligations, for any reason whatsoever, including without limitation the fact that (i) the Guaranteed Obligations, or any part thereof, exceeds the amount permitted by law, (ii) the act of creating the Guaranteed Obligations or any part thereof is ultra vires , (iii) the officers or representatives executing the Note, the Mortgage or the other Loan Documents or otherwise creating the Guaranteed Obligations acted in excess of their authority, (iv) the Guaranteed Obligations violate applicable usury laws, (v) the Borrower has valid defenses, claims or offsets (whether at law, in equity or by agreement) unrelated to the willful misconduct or gross negligence of Lender or anyone acting by, through or under Lender which render the Guaranteed Obligations wholly or partially uncollectible from Borrower, (vi) the creation, performance or repayment of the Guaranteed Obligations (or the execution, delivery and performance of any document or instrument representing part of the Guaranteed Obligations or executed in connection with the Guaranteed Obligations, or given to secure the repayment of the Guaranteed Obligations) is illegal, uncollectible or unenforceable, or (vii) the Note, the Mortgage or any of the other Loan Documents have been forged or otherwise are irregular or not genuine or authentic through no willful misconduct or gross negligence of Lender or anyone acting by, through or under Lender, it being agreed that Guarantor shall remain liable hereon regardless of whether Borrower or any other person be found not liable on the Guaranteed Obligations or any part thereof for any reason.

2.5            Release of Obligors . Any full or partial release of the liability of Borrower on the Guaranteed Obligations, or any part thereof, or of any co-guarantors, or any other person or entity now or hereafter liable, whether directly or indirectly, jointly, severally, or jointly and severally, to pay, perform, guarantee or