I made an FDG Siege Tower, complete with hard wheels, and a textured base. It was nice -- until it got banged up, and bent. I've had the same issue with castles, in the past. I needed to find a way to make them more durable, as well as heftier, so they would not blow across the gaming table with any light breeze.
Enter 2mm-thick Backboard, from Michaels Craft Store. It is sold in the framing area, where it is sold as backing for frames. For the Siege Tower, I glued two layers of Backboard together, then I used a glue stick to attach regular paper printouts from my color laser printer, to the Backboard pieces, glued them together, and back to the old base. The only issue, was that the thickness created mating problems: things did not line up anymore, due to the thickness, leaving white areas along the seams and edges. At first, I tried using a black marker to cover/darken the edges. I didn't like that, so then I tried painting them with gray craft paint. Voila'! Here are some shots of the new model, and the original, 110# card stock model:
This shows the gray edge painting in progress, with the right side edges unpainted yet.
This photo shows the old model, made out of 110# card stock. As you can see, the card stock models are not very durable. They also are quite light weight. Next is a photo of the top of the new, heavier model:
Here is a link to a slideshow, featuring many more photo's of both towers. Thanks for reading. I hope this inspires you. Cheers!