Gratuitous Tank Battles can be an incredibly fun and rewarding game. Remember the fun it was to build your own custom ships on games like Master of Orion? GTB is a tower defense game that's like that for every single unit or turret. From the smallest cheap gun to the big strongholds, from inexpensive recruit infantry to heavily shielded mechs.
Designing the units is simple, but the management of new and older designs can be a little busywork. But this kind of complexity that usually lacks on modern games is very welcomed.
The very fact that this games plays as both a traditional tower defense game and an inverted tower defense game is a testimony to its flexibility. It's a robust selection of units with great variation of strategies you can use to beat the enemy.
At least for me, the fact that you can design, customize and name every unit in the field adds to the enjoyment and creates a very fun experience when your guys are winning a battle. It's just most interesting when that tank unit you built to be fast and strong barely makes it to the finish line.
Building units is not everything, and the battles are intense and complex.
Apart from the built-in campaign, you can edit your own scenarios, and upload and download challenges from other users.
If you like Tower Defense games or games with customization, this is a very good choice.