There is a bug that makes the game a lot easier than I think it's meant to be. If you sell a wall, it will override the duration of your current cooldown with the super short wall cooldown. This lets you upgrade all your towers to L5 super fast, you just go Gatling -> Sell wall -> Gatling and you have it at L5 faster than you'd normally have it at like L2 to L3.
On that note, gatling is kind of crazy overpowered due to its attack speed. +1 Damage upgrade just works so infinitely much better on it, than anything else. That a single Gatling L5, that you funnel all upgrades onto, just solos waves.