The Armamentarium is Delft's former arsenal, a substantial historic building near the old centre and canals. Even when visited from outside, it adds military and civic context to a city better known for Vermeer, churches and blue-and-white ceramics.
Its use has changed over time, so the reason to seek it out is mainly architectural and historical rather than a guaranteed museum visit. Include it on a canal-side walk between the centre and the southern edge of old Delft.