TD game.

[Bhaliar]

Okay, so I'm gonna attempt to make a TD game on the graph screen what it will feature is

Sprites for towers and enemies( Big step for me. My first attempt at sprites.)
The towers won't move.
Enemies only appear one at a time.
Health, money gain, and possibly tower upgrades.
Sandbox mode.
4 Tower types.
-Ground Tower
-Air tower
-Ice Tower (Slows enmies)
-Super Tower.
5 enemy sprites that are cycled through over and over again.

I only have one problem I need some help on.

I have no idea how to do multiple towers. I can make an engine for placement. I can make an engine for damage and all. I just have no idea how to make the game keep track of multiple towers. And by keep track I mean to make sure it doesn't get erased, make sure they all are active and not useless.

Yes, this may seem much for me, but I want to push myself. For the multiple towers, I guess I could use multiple different named lists for each one. And range of towers I've got to. It's just the whole how to keep track of every range while still having enemy move.