I'm about to make buildings e.g. castles and underground tunnels, for my game levels. Would you recommend using Autocard or archicard. If you know any software that can make this easier apart from the ones I've mentioned please tell me. Thanks.

I would recommend 3ds max / Maya for your game levels. AutoCAD/ArchiCAD series are generally for architects and professional designers (engineering).

It really depends on what kind of file format your game is going to read levels in. If it's going to be more of a polygonal model rendering format, you should use something like 3ds Max or Maya, or something like blender or milkshape on a budget. If you are going for a csg-style environment then you should use something like quark or some of the other free csg level editors out there.