You could register the click in the scene behaviour (not the tower actor behaviour).
Something like this
if mouse was pressed
{set y to scene-height
for each actor of type towers
{if mouse was pressed on actor
{if y-position of actor > y
{set y to y-position of actor
set clickedActor to actor
}
}
} trigger event for clickedActor (to open menu or whatever you want)
}
What this should do:
1) work only when mouse was pressed
2) check each actor to see if mouse was pressed on the actor. If it was, check if this actor y the lower then any other clicked actor (as you first compare to screen height and later to any other clicked actor)
Another, maybe easier way this could be done is by creating additional, square, invisible actor (in create event for tower actor behaviour) and checking if it was clicked. This square actor would need to have physics disabled (as I guess you also disable physics for tower actors). The downside of this method is that the player would always need to click on that square at the bottom of the tower, while the first method would allow to register also clicks on the top of the tower, if there is no tower on the higher tile.
I guess yet another way would be by creating a region and checking if mouse is inside that region, but I haven't really tried that.