[Trigger] Only attack one type of unit help

[Chikokishi]

Im making a TD, and if you look at the picture, im going to have towers randomly spawn in the grid of small squares, and in between rounds ill have creeps spawn in the two larger rectangles. this is my bounty system. Anyway, i want the towers to ONLY attack the creeps that come into their areas. In my case i have a chicken spawning there. but my towers insist on attacking creeps that follow the path on the bottom of the screen, and iv tried everything i can think of to make it stop. There is an event called "unit acquires target" that i thought i could use. but it needs a variable that i dont know what type it is, and i cant select the building because they spawn during the game. Can someone help me?