A friend of mine in the local gaming club has put together a Kickstarter for a project he has been refining for 12 years. I present to you Castle! by Castle foundary!
http://www.kickstarter.com/projects/castlefoundry/castle-the-modular-construction-system?ref=live
Major selling points of Castle!:
Highly modular, can be disassembled and reassembled as many times as you wish in any configuration you wish.
Durable, these pieces are weighted like stone but as resilient as plastic.
Delaminating, you can quickly reach any level of your model castle for fights ranging up and down or as you break in to the walls during a siege, etc.
The plan is to get funded and at some point expand out of fantasy castle by making different "skins" as it were for modern, high tech, and to expand the castle's themselves, there is discussion of making replicas of real world castles! Something I thought was neat.
Having seen these in person, they are very sturdy. I'm 6 foot 6 and we were able to build a wall that was in no danger of toppling all the way up to eye level, so the structural integrity of these things is pretty darn impressive. Check it out if you are looking for a good looking fantasy terrain kit!