We were delighted to discover this tiny gem. There are just two very small floors of two rooms each, but they were full of interesting information about the original Palace of Westminster and the history of the building. Whilst perhaps too small to justify a trip into town just for this, I'd definitely recommend a detour if you're in or near the area. There's also a small cafe on the ground floor. As others have noted, this is a small building with most of the space accessed via a narrow, steep stone spiral staircase.

An English Heritage site. Original tower dating back several hundred years. Lots to learn about & original artefacts on display. Coffee shop on ground floor, manned by very helpful staff.