Z-Order

I'm making a tower defense game, and I want towers to overlap each other. So a tower with higher Y position should be ordered 'on top' of a tower that is above it

Eg:

<img src="http://img18.imageshack.us/img18/4122/towerhelp.png">

For each object(tower), ordered (every tick).

By object's(tower) y coordinate.

Ascending

Object(tower) send to front.

Thanks for the help