One funny thing about the UK is that it happens to be very old. That means that a lot of the buildings people still reside in, use for school or business, etc. are also very old. Unlike in the US, where as time passed, we just built our "normal" buildings separate from the historical ones, the UK just tended to retrofit and fill inwards. This means you get Burger Kings directly across from castles which are snuggled right in the heart of town. This is where we find ourselves with Cardiff Castle.
Caby and I went on our own for this one. There's actually two parts of it, the main lodgings (where we went first and thus is what you see first in the gallery), which is incredibly ornate and a museum as much as it is a historical building, and then a shell keep which is more stony, barren, and more what you'd expect from castle ruins. There's also a super fancy clock tower, but we didn't go in it, unfortunately. I think it was closed when we were visiting.
Believe it or not, Cardiff Castle is actually still used outside of the context of being a historical building. Concerts as big as Tom Jones and Green Day have been had within its walls. Perhaps we'll catch one on a future trip?