Steampunk Tower is a perfectly competent tower defense game. Instead of building towers, you place turrets into a big tower standing in the middle of the battlefield. Enemies attack you from left and right and you can move turrets around to upgrade them or to reload ammo. Enemies are weak against some type of turrets but strong against other, forcing you to build a wide array of defenses, you can upgrade them between fights, using oh-so-precious dollars. It wasn't the best tower defense game I've played, but I enjoyed it.
There's not a lot of variety in the levels beside the challenge maps (where you have limited resources and sometime low health), each level is just wave-based survival where new enemy types are added as you go through the game, maybe mix it up with other type of levels, how about having to fight another tower? Or having two towers? Or having only turrets on one side.
To be able to level your turrets, you need to research stuff between maps, this costs money that trickles down quite slowly, you can grind maps to get more of it, but that's not fun, I have nothing against buying permanent upgrades for your towers, but having to buy the potential upgrade paths make it difficult to have a diverse tower choice; In my situation I had a few level 2 turrets, but my buzzsaw canon was still level 1. This doesn't ruin the upgrade system, but having to buy the things you're going to be allowed to buy later is a bit silly.
And in battle, you have a different currency to buy and upgrade your turrets. It gets completely useless as the fight goes on because all of your tower is filled with max-level turrets. Give me something to spend my money besides turrets and in-fight upgrades, maybe refill my ammo in exchange of cash? Replenish my special weapons? Allow me to upgrade to a higher level by paying premium? The special powers are things you get for real money or slowly by beating missions and they deal huge damage, sometimes I even wondered if they were required to beat tough spots with tons of tricky enemies without grinding to boost my turrets. But then I beat the last level by burning all the special powers I had hoarded through the game, and that was that.