The area effect towers fire at nothing so often that they are always reloading and missing targets because of it, I get cool down time is needed but they need to stop randomly burning empty sections of track and only activating when something is in their range, also the amount of funds earned seems completely unrelated to performance and is given at the end of each round, leaving no chance to upgrade or modify defense as attacks come in. The difficulty also jumps up and down so wave 3, when you have almost no towers yet, might be a swarm of dozens of hard to kill bad guys while wave 8, when you have plenty of defense might only send one or two out.
The cost of upgrading or combining towers is virtually impossible to meet with the low income, generally a first tier upgrade shouldn't cost as much as a whole new tower and combining should be practically free since you already have one tower, you should just be able to buy and place the new tower over it for very little additional cost.
Some kind of type guide for the bad guys and a way to sell towers that are placed accidentally would also be quite handy.
This game has a lot of potential and I love the retro graphics and the combinations but it really needs some of the issues ironing out.