I agree on Acheson. I love their Frontier America line. Here's what mine looks like all finished:

The pieces you see with stairs are converted ones. I took the piece you see next to the tower in the top & center of the photo and created my own stairs with bass/balsa wood.
Acheson also did a smaller fort:

Great stuff! If you are able to find someone selling the pieces, I recommend running them through the dishwasher on low/no heat to remove the mould release agent. Good luck!
Mike Demana