Any fiefdom can only be attacked by one alliance at a time. So if you and a neighboring alliance make a pact, you can attack each other’s last fiefdoms, with no gaps in between, only participate in your defences, and can survive indefinitely. You have a 50% chance of chance of getting 3rd place each season, even though you lose 2 fiefdoms.
so how do you go about contacting the other alliance? Unless they have an obvious Facebook group or similar there’s no way to get to them and each season it’s a different alliance you need to get in touch with.
Send an ambassador, of course.