Tower issue;
The only multi-shot tower is also the best tower.
I did 25 levels in a row using just the Multi-tower and always got an Expert score.
I would like to see the basic (and least useful) tower replaced with a long-ranged weapon that covers a lot of area.
Gas tower caused auto-fail. I never use the frost towers due to the effectiveness of multi-tower.
Bombs weren't something I used either. I think it would be better if the bomb-upgrades included a freebie upgrade. One free bomb per level?
A high-damage low-range (melee?) tower would be an interesting addition. Like an eletric fence? Hits every unit just once each.