Heheh, yeah, Deus Ex ladders are death traps if you're not careful and don't understand how they work. (Same was true for System Shock 2, in my experience.) It's really just a narrow volume of space in which you can move in any direction - generally you want to face the ladder when you're moving or you'll almost invariably fall out of that area. The trick to getting onto a ladder from the top is to back out over it, then go down by looking down and moving forward. If you fall off backing up, just hit forward and you can grab onto it.