On Governors Green, south of the Royal Garrison Church.
This mortar was formerly located some yards to the east on the site of the demolished Governors House. At that time a plaque was attached to the platform. This is now missing.

Government House Mortar
Inscription on the Former Plaque (From a guide to the Royal Garrison Church)
On this site stood the old Government House of Portsmouth. It was on the visit of the Allied Sovereigns in 1814 that the Prince Regent, The Emporor of Russia, The King of Prussia, The Duke of Wellington, Marshal Blucher and Count Platoff, Helman of the Cossacks, surrounded by a host of Princes and Warriors, came forward on the balcony and drank to the prosperity of the British People.
This Mortar and Platform were here placed by Lieut-General Lord Frederick Fitzclarence, G.C.H., Lieut Governor of Portsmouth 1848, and restored by Lieut-General Henry Kent, late 77 Duke of Cambridge Own Regt. during the Lieut-Governorship of General Sir George Willis, K.C.B. 1889