Place the loon on top of the tower in the mission editor. In his init field write:-
this setpos [getPos this select 0, getpos this select 1, 10]
Syntax not guaranteed. Experiment till you get the right number - it maybe, say, 8 or 12 rather than 10.
Depending on what you want to happen later, you may find some of the following commands useful
setUnitPos
doStop
disableAI