The top part of the main cylinder (those box-shaped thingy) should be sized up. Look at the concept drawing, the box thingy with the triangle support underneath together should cover half the length of the whole cylinder. Yours is about 1/3 for now.

The turret on the right with the walk way should be lowered to just below halfway of the cylinder. The top part of the turret ( not the squares) could be bigger, just a bit smaller than the squares.

Thichen the polls around the cylinder, narrow and lower the entrance/exit and you should be pretty darn close.