Here's a hint, it doesn't matter what unit you build. All units survive 1 encounter and then die. You can easily get stuck in a near endless back and forth.
Game plays sloooow. 120mph doesn't feel fast. The collision detection is awful so your car hits a lot of stuff you'd think you missed. Just not very good.
You can do the same thing the same way multiple times and get different results based on random falls and drops. It isn't fun to do the right thing and still fail.
On some levels you need to get the "right" towers to shoot first at the ninja bosses - namely freeze/fear towers. Be sure to have one by the entry and then sell any other possible "first shooters" so that tower gets the first shot off.
One other thing, right now all towers are sort of the the same. I could just build all of one type of tower and the game would be basically the same. It'd be nice if Orc Towers could "see" ninjas while the Fire Towers didn't miss Dervishes or something like that to make 'em feel a bit different.
Greta game 2 issues: 1. Targeting heroes with gems. Too often my towers won't fire on the highest value targets. 2. Ninjas, 5 seconds is way too long to go invisible on maps this size.
I so hate that juggling is considered a necessary part of all TD game. They're strategy games and not reaction games. Stupid feature for lazy developers.