Good question about the top.

1.)  The top was not shown really in the movie (Star Trek the motion picture).

2.)  I didn't put anything up there either.

3.)  I hadn't really planned on rendering it from above since the interesting stuff is down below.

4.)  Thrusters are of course logical but they weren't really in the movie.  That doesn't mean they can't be added though, this is my version of it anyway and in the end, it's all just art.

-Kix