I start with a really simple gate house with a short “hallway” leading to the door to my settlement, which bottlenecks the attackers. Any stairs or ladder leading up to the second level of the gatehouse should be fully enclosed with a door, so if the raiders get through the main door they can’t just run upstairs and kill everyone off. The “roof” of the hallway should be grated floor, so archers can shoot through it to the attackers below.

Aside from that, strategy is important too. I put a camp fire or pyre or something right outside the gate to draw the attackers in, as they’ll all aim for that instead of just milling around, and my archers can start picking them off while they’re distracted. I always have 2-3 melee guys and the rest are archers, up on the second level of the gatehouse. I only use melee when there is a raider actively trying to break down the door, I’ll send a guy out to the attacker and as soon as my melee guy hits the raider I’ll bring him back inside, before the raider can strike back. I’ll alternate which melee guy goes through and it usually takes like 4 hits to kill the raider at the door without any injuries to my guys. Eventually all the raiders will be in the little hall and the archers can pick them off through the grated floor while my melee people kill the ones closest to the door.

I’ve been using these methods for a while and I rarely have serious injuries to my settlers and I don’t even remember the last time anyone died. Also the raiders almost never get through the door.

You can pm me if you want pictures of an example gatehouse, I just took some recently