In Evony, the Embassy hosts military units from your other alliance members. You need an Embassy if you want to create or join an alliance (level 1 to join, level 2 to create). An alliance host may increase the maximum capacity by upgrading his/her Embassy. For people who want their alliance’s military support, the level of the Embassy allows that number of armies to be stationed in the embassy’s city regardless of the army’s size. This means that if you have a level 7 Embassy, 7 of your alliance members may assist you with however much army they want.
Sending a bunch of your troops to reinforce an ally’s city will leave them there to fight and defend the city. The owner of the city may not control your army and you may recall your troops at any time. Food will be consumed on the owner’s part as if your army were his/hers.
You have to select the “allow troops of allied members to garrison your current city” option in the embassy to allow your alliance teammates to support you.