Michael
I saw this set you could order online using mold. It actually looked pretty cool...

naebles
If you have the time, I would use salt, flour, water dough (you can google to get a recipe you want). This method is very inexpensive, but if you put some time into it, you can really make your project great! It requires some drying time (in the oven or air dry - but oven is the fastest) - this is to your advantage though! If it were my project, I would make "bricks" out of this dough and add food coloring to the dough before baking it, also any flags or a bridge in the same way. Then construct the castle itself. Next, I would make more dough and coloring it for grass and water, maybe even some random boulders - this would surround the castle (I would let this part air dry). I remember making a topographical map of the United States with this stuff a while back, it was really easy to work with (shaping and what not) Good luck and have fun with it!