this needs a serious balance pass, tried every possible strategy available early game but you simply can't catch up to the spawn frequency and enemy health scaling and get very rapidly overrun. dont think i've gotten more than 2 towers with crank-powered before i died each time. the base towers simply suck too bad and the xp upgrades costing money leaves you in a downward failure loop.
also, the aiming ai is bunk, the few, slow shots each tower CAN get off before enemies are promptly out of range tend to be wasted on overkilling the first target, letting the next 2 enemies behind that overkilled guy just continue on down the lane uninjured.
maybe upgrades help this problem but that is a moot point since you don't generate nearly enough gold fast enough to get out of the downward failure loop mentioned prior.
The highest I've gotten is wave 30, and that was with the strategy of using 5 turrets and spreading out upgrades evenly between them. Which results in me only being able to level them up once (50xp + 50g each) and being able to spend 150g on crank-action upgrade for just 2 of them before I die.
not investing in the first level up to save the 250g cumulative gold causes you to get overrun around wave 15-18 as your turrets can't even kill one thing with their abysmal fire rate, combined with being no longer capable of one-hitting red blips anymore.
All the upgrade options are there, but at the current state, the player never gets a chance to pick them up.