Unlike the game's predecessor, War Commander, turret levels are separate between Defense Platforms after the Defense Lab was removed before the worldwide release. Like the Minefield's mines, equipping or upgrading turrets do not give XP.
The best and most common strategy is to group them up into one area to maximize their turret's DPS covered with Minefields and Barriers. Your opponent will get no stars if they only destroy your buildings other the platforms.
Turrets must be individually purchased and upgraded, and each ix exclusive to one turret only. Their fire rates can be buffed with transformers. If a defense platform was destroyed the boost to other turrets of a transformer will increase, if multiple ones are on it's AOE.
They are not to be confused with heavy platforms which are much more durable, have a different selection of turrets, and are buffed by Heavy Transformers instead.
A platform will be unarmed at first unless the player purchases a turret for it, upon which it will become weaponized for the rest of the game.