Ehrensvärd Museum is located in the original central square of the fortress, in the Commandant’s House. The first host of the building was the founder of Viapori, Augustin Ehrensvärd. The beautiful rococo interior showcases, with the peculiarities of Viapori, how the nobility lived in the 18th century. Portraits, style furniture and models of ships of the archipelago fleet are on display.

The Commandant’s House is a good example of how living quarters and defence structures in Viapori often merged under the same roof. The building was the living and official residence of the Commandant of the fortress until 1855 when it lost its entire southern half in the Crimean War bombings.

In 1927, the Ehrensvärd Society took the initiative to establish a museum in the rooms of the Commandant's House that had been the residence of Augustin Ehrensvärd. The museum was opened in 1930 and its exhibition was renewed in 2015.