If you're trying to make a TD game don't. GS cannot quite handle this yet. Constraining all those X/Y's of the enemies will kill FPS. Plus having several 'towers' spawning or even recycle 'bullet' units will kill it. Then the sound that the collision with enemy has to make. Throwing out 5 sounds at once will kill performance. Then you have to track the damage each type of tower will give to the enemy, this will take a lot of rules in each enemy actor.. Trust someone who has tried, hold off on TD games for a little while.
Comments
NOTE: You should also be constraining the actor you are shooting at's x and y variables to global variables.
vector_to_angle(position_x - (actor's_global_x), position_y - (actor's_global_y))
There are also a few examples on this, just search in gamesalad templates.