The Alliance Gate allows you to create or join an Alliance.
It initially appears as a ruined building and it can be repaired starting in the Iron Age in order to unlock its functionality.
Upgrading it increases the capacity of alliance troops.
Unlike the standard buildings, you cannot move it, and connecting it to the Town Center via Roads does not grant a Road network bonus.
The Alliance Gate cannot be destroyed in battles.
You can only send a maximum of 6 individual units to each ally at once per request. For example, if your ally sends a request and has a troop capacity of 12 troops, you could send five Heavy Infantry and one Heavy Cavalry, but you couldn't send seven Heavy Infantry units.
The French can receive more donated troops from Alliance Members (+3 more donated troops). This becomes +6 more donated troops in the Gunpowder Age.