My route:
Up the elevator (no other way as far as I'm aware), to it's back door, onto the small tower, from there onto the metal bar jutting out from the wall and then onto the roof, across it and back down at the front.
Instead of going to the gazebo, look to the right and go across the metal roof that leads to the edge of the wall. You can't reach it directly, so blink to the posts of the fence/wall. Careful though, there are two guards below, usually near the stairs. They can see you from certain spots. The Tallboy patrolling below can spot you from certain angles, too. Now over the wall.
You're in view of the watch tower and one guard patrolling up and down the stairs of the smaller building right ahead (he can be hard to spot because of the bushes, but you'll hear him whistling), otherwise the area is empty. Keep right and go along the path to the its end, eventually the wall on the left will reveal a way up. You're already on the right side of the building at this point. Watch out for guards on the way up the scaffold, but don't go all the way to the top (someone's patrolling there and there's no need). You can blink to the overhang of the building from the level below it, then follow the overhang and you'll end up at the same hatch, I believe. Once you're done inside just head back the same way.