I don't think turrets are absolutely necessary, but using the strategy of having a long unubstructed path to herd enemies down a predictable path is.

I have a bunker on the front of my base that leads into a long unroofed hallway with ruble that opens up to a fighting position (like a small bunker), behind which is another fighting position supported by 5 turrets. My first bunker has auto doors on the sides which let my guys in it be able to quickly get around the first hallway.

I try to outfit my guys with sniper and assault rifles, letting me do a ton of damage from my first bunker before they even get in range. If it gets to mid range I move my snipers back to the last fighting position and let my rifleman keep fighting. When the enemy gets close I move my rifleman to the snipers through the autodoors, and this time they are making their way slowly through a long hallway as my snipes are shooting at them.

If they make it to the fighting position my guys can go back to the courtyard and have help from 5 turrets to finish off whoever is left.