Tower Owning trigger?? (Complicated??) Help plz

Ok. Heres the scenario: You can get 2 towers created in a location. If the towers are created, you can get a tower engineer, bring it to the tower and take ownership of that tower. Now, how can i do this if the tower isnt there already?! In the triggers its asks for a unit to click on or a variable.. any ideas guys? Eather getting close to it or attacking it would work fine.

Damn.. any1? This thing is drving me insain!

You need to some way or another put the towers into variables. Yeesh. Create 2 variables, Tower1 and Tower2. Then do this:

Create 1 tower at blank
Set Tower1 = Last Created Unit
Create 1 tower at blank
Set Tower2 = Last Created Unit

Now Tower1 and Tower2 will show up in the variables list.

What type of varable should they be? Im not very good with varables yet.. =(

....Units....