I did that once with a foam castle, and the kids had to build siege weapons. Well the high schoolers did some pretty destructive designs, but all failed, save for a kid who brought in a tongue depressor and a light, rubber band powered ballists. He said he had plague, stuck the depressor in his mouth and fired it into the castle. It was covered in slobber, and no one wanted anything to do with that castle after that. I awarded him the grand prize!!!

What a great exercise for them - lots of good engineering and physics concepts in play there, as well as team work and pure, downright invention.

You could always get them to sit down and work out, from what they've observed and what they know, how to beat the castle.

As for an easy solution: water will take the castle apart fairly quickly. I'd suggest they try firing wads of wet paper - they'll have the satisfaction of seeing their wads sticking to, and eventually overwhelming the castle.

I like the idea of using water - although some of the water may be from my tears as my castle disintegrates!

I was thinking of wheeled battering rams that could be rolled down a shallow slope into the castle.

Also there must be something i could do using the siege tower from siegeworks but I have to build it first before i can get my thoughts framed right.

I had also been thinking what other design challenges I could set using other worldworks sets - moonbases with Mars station seems an obvious idea, sea battles with Skull Cove/Maiden