Fixed gun ports would be a nice and slightly earlier game alternative to the turrets, this is how i think it would be nice if they worked.
First of all there are few or no vehicles with fixed gun ports at the beginning of the game, you construct them with any ranged weapon at hand (which has the possibility to be loaded with ammo beforehand, so no bows or slings, but a crossbow would be possible) which gives a lot of choice concerning weapon types.
They only fire when a command is made at the controls, not automatically when an enemy is presented.
They are aimed at installation, similar to headlights and will be “aimed” at the furthest point in that direction which the weapon and ammunition allows, this should also mean that they are about to likely to hit as crossbow- and shotgun-traps and not really foolproof unless used with high rate of fire and/or area weapons.
To reload them, one has to access them directly, so for example using a crossbow gun port you can fire once and will then have to leave the safety of your seat to go out to reload and arm the crossbow.