Ensemble Studios Online 2 (also known as ESO) is the matchmaking service in Age of Empires III and its expansions. Requiring the creation of an online user account tied the user's product key, ESO features a quickmatch option, where players are randomly sorted into games. Players can create games and play in a game of their choice via the game browser.
A player's online Home City is separate from their Skirmish Home Cities. However, if there is a Skirmish Home City above level 10, then all ESO Home Cities will start at Level 10. If a player's Home City starts from level 20, then adding by each 20 levels, a newly created Home City will start on that level (e.g. current Home City level is 40 = new Home Cities begin at level 40).
In ESO, players are ranked on their skill level. The ranking system employs the ranks used by the USMC. The ranks are as follows: