To become Allies with other players, you send an "Alliance" offer for other players to accept. If they do, you become Allies. Your allied status comes with certain benefits - Reinforcements and Caravans travel faster between allied Castles, and your Counselors will get reports about your Allies' actions. To form Alliances, build an Embassy. Your Herald will report new offers and inform you every time an Alliance has been formed or terminated.
You can become Allies in the following ways:
You can't raid or lay siege on your Allies' Castles, however you can raid any Towns they currently control.