There are 3 types of Alliance Relationships: Union, Friendly, and Enemy.
Both members of a Union cannot attack each other, and they can gather resources in each other's territory without paying tax.
Union alliance territory will be marked in blue on the world map. You can use Territory Teleports within your Union's territory to teleport cheaply!
Friendly alliances will be marked in tawny on the map.
If you try to attack a Friendly-relationship target, you'll get a pop-up reminder asking you to confirm before proceed, please think twice when you do so.
And last one Enemy alliances will be marked in red on world map, be careful when you see them. They might attack you at any time!
After setting an alliance relationship, both sides will get a notification mail.