Your embassy can be important. You not only need an embassy to join an alliance but it can also help you defend your city.
Based on the level of your embassy you can support armies from your allies. A level 3 embassy will allow you to host 3 of your allies armies to help defend your city. Your allies armies only help defend your city. You cannot control them.
When you host your allies armies you need to feed them. Make sure you have lots of food. However food is the cheapest resource in the game. If you follow my advanced strategies you should have a ton of food and not need very many farms.