Sorry, but I have to say that turrets are too powerful.
Along with bear traps, they cost 0 and that is much too low ;) . Plus they shot with a little delay.
Making a base that is impossible to defeat (for sure in early game, I don't know where is "late" for now) is very easy. Enemies are trying to infiltrate your base, so leaving open doors with turrets behind them is the easiest way to kill them without any problem.