TimeRef is a great source for general medieval and middle ages history in general, with not only a bunch of info on castles (sadly not the floor-plans I was looking for), but a good amount on general life in that age as well.
What really drew me to this website was a large collection of 3D castles, keeps, and general buildings. You'll need a couple different plug-ins to use them, but they are really worth checking out.
Using my computer to walk around "A Medieval Stone Keep" was almost like playing a video game, but there was a serious lack of furnishings. My initial thought was it'd be cool to take screen-shots and then dress them up as player props......but I couldn't take a screen shot.
It might be cool to use this with my TV to have the players navigate around the keep, or maybe just use my laptop and my projector. On the right surface I could draw my own props or even use my own monsters to create an interesting one-off adventure. Project onto a magnetic whiteboard and slap my own magnets on there.
Actually I think this would be a ton of work for a one-off, but as a re-occurring convention game, maybe a storm-the-castle learning game? Some serious potential.
At any rate, I think you should check out TimeRef and have some fun.