An alliance is a group of players on Backyard Monsters that have the same interests, style of playing, and are united together in case one of them becomes endangered. Usually the person who created the alliance makes the rules and is the Alliance Leader. Some well known alliances can be found on the forums. Alliance can contain from 1 to 50 members (the current limit for members of an alliance is 50) and those that are helping each other out in all sorts of other ways. Alliances may take advantage of the Alliance Power-Ups.
To make an alliance you need a Map Room Level 2 so you can access the World Map. There is a tab that says Alliances at the top (if you have Map Room Level 2). Click on it and click "Create Alliance" and you are on your way to making an alliance!
In this tab, you're able to see the top alliances in your world (server) and the whole world. You are also able to search for a certain alliance and join them. If you are the alliance leader, you are able to flag other alliances as Hostile, Neutral, or Ally.
Here, you see your alliance's name, logo, rank, level, leader, members, and alliance chat box/log. Here you can leave the alliance or post a message in certain situations, like if you are under attack by a hostile alliance or just feel like posting a message.
Here there are different power-ups. These are Armament, Conquest, and Declare War. Only the leader can activate a power-up, as long as it's ready.
You will see all of the alliance's members here. You will see their level, status (if they're online or not), their empire points (the value of the main yard and outposts), and their last attacker. You can also visit their base through here. If you are the alliance leader, you are able to promote any member to leader or remove anyone from the alliance.
If you are not in an alliance, you will see here if an alliance is inviting you to join them. If you are already in an alliance, you will see here who wants to join your alliance.