It's hard to get him to climb AND take the right position. :-\
But if ya use one of, say these latests "baracken" towers (the addon - ya know it?), then it helps to setpos a static M2MG up in the tower and then let the AI moron climb up & get in the MG.
As for AI taking position in other towers (smaller towers),
I'd recommend ya to simply do like this :
Name the guy Bob in his name field.
Make a WP for Bob and put it right in front of the tower (or directly upon it).
The in the "On Activation" field write :
this action ["Ladder Up", Tower1]
Now, that will make him climb but then ya need a trigger/scroll with this code:
==============================
~4
Bob setpos [getpos Tower1 select 0, getpos Tower1 select 1, 5.2600]
==============================
or alike to setpos the guy on the right spot in the tower
this should fire when the WP is activated
(it needs a delay to give Bob the time to climb the ladder.)
Oh and P.S.
off course, ya need to name the tower "Tower1"
;D