The Great Courtyard used to be the administrative centre of the sea fortress. Augustin Ehrensvärd, the founder of the fortress, was buried in the square.

Completed in the 1760s, the Great Courtyard was the first monument square in Finland. Obliquely positioned bastions and arched buildings created a false perspective in the square. The optical illusion was used to highlight the administrative centre of the fortress where the main guard house for the fortress and the fortress commandant’s residence were located.

The Great Courtyard was badly damaged in the Crimean War in 1855 when the Anglo-French fleet bombarded it. The Bastion Ekeblad, a part of the Commandant’s House, the General’s Building and another arched main guard house building that were damaged during the Crimean War were demolished. A residential building for Russian officers was built in the square.

Today, the courtyard houses Augustin Ehrensvärd’s tomb, the Ehrensvärd Museum that is open in the summer, and residential buildings.