I just got the Pegasus Gothic building kit and I'm very impressed.
What is it?
Pegasus Hobbies makes a line of terrain and figures.
http://www.pegasushobbies.com/gm_main.htm
Their gothic building kit is $30 for a reusable sturdy plastic building kit. It's clearly meant for 'gothic wargames' but is not so over the top, it can be used in most settings.
What do you get?
12 x 3" wall sections
6 x 6" wall sections
1x 3" door section (openable)
Gargoyles, torches, butresses and other decorations.

What does it look like?



How does it fit together?
Very, very well, the tower took just a minute or two to assemble and came apart easily.
What would make it better?
There's no roofs or floor, ladders and stairs are also missing. Roofs and floors I can do with plasticard and I can always buy some ladders (or just assume they're there) but it is a bit of a bother.
Bottom Line
Cheap, easy to use, looks good. 9/10