York is one of the few cities left who still has its city walls. Bits of the walls date as far back as the Romans, but most are medieval. They're in great shape and you can walk all the way around the city on the walls, including going in at least one fo the towers, which contained a small museum.
If you're looking to build some medieval terrain, these should be a handy reference!
This is York Castle - which dates back to the Norman conquest, and is an example of a motte and bailey style castle. It is surprisingly small inside.
These last two pictures are the ruins of an Abbey (if I recall) that we visited. Great reference for ruins, and weathered stone!