Ok. I set some neutral houses, four of them, so there is one for each player. I have set a trigger "Make Allie" that will start to wqork when teams build certain buildings. Ok, that didn't come out too clear, so heres an example. Team four starts at th bottom left corner of the map. There are neutral house defences at the entrance of the base. Say, when player four builds a power plant, the neutal defences near their base will become friendly towards that player. The defences will not attack that plaer but will attack other players.
Like I said, I created four more neutral houses, one for each player, so they may have "preset" defences for themselves.
I have tried this several times on different maps, but it sometimes doesn't work.
Player starts at a spot, they build power plant, the defences become firendly towards that player.
How I know this, well, it was an accident. I set triggers, where when anyone builds a power plant, the neutral house becomes friendly towards them. Then, when they build ammo crates, (Yes, I made ammo crates buildable) the neutral house becomes hostile towards them. I don't build them for that reason. It's really fun when you have the neutral forces act "friendly" towards you.
I hope that was clear enough.