how build fort
Get some suitable packing polystyrene, cut off anything you don't need and make a fort out of it. Often they look fort-ish.
Then use an undercoat sealer and then spraypaint. Don't spraypaint w/out the sealer as the spray stuff eats the polystyrene, and then your fortress will look like it has melted.
Failing that, buy one from the numerous hobbyshops out there.