Go to the ladder that leads to the very top and face it while standing on the ground.

Turn right and walk into that small area. Turn left and jump/climb up that block.

Now, you can camp there, or turn right and jump out onto that area, or you can go forward and be on a platform that is fairly hidden and is directly over one of the passages that go through the main 'building'.

If you choose the forward option, note that you have to stay to the left as you climb into that area, the game often won't let you climb in from the middle of that starting 'block' (I think it curved on top which stops you from climbing the metal bar unless you scoot left a little).