Is there a program that lets you build 3d castle simulations?

[MonkeySage]

The closest I've come so far was Medieval Engineers, which hasn't really worked out so well; the game has pretty severe limitations and it's kind of glitchy.

[Knaight]

What exactly do you mean by "simulations"? If it's just 3D models, then there's a number of good options (Sketchup Make being a free one with a learning curve that isn't too terrible). If you want physics modeling of a castle under siege or something, there aren't really any good options. If you want a video game that does sieges where you build castles, Medieval Engineers or similar is probably your best bet.