You will find a good exhibition about it's use as a prison up to Victorian times, including those on death row.
You will find an exhibition on the Magna Carta as well, although for more details see the tip on it.
You can also walk the walls, which is great for photography with impressive views over the surrounding city and pancake flat Lincolnshire countryside.
I like the fact that although the present castle has it's roots in Norman times, it has never ceased to be a working castle, rather than being 'preserved in aspic' like so many others. The largest building in the complex is actually the Crown Court for the area.
Nice little cafe too.

