The Consulate allows you to host and send reinforcements. It also grants a bonus to the health and strength of reinforcements that you send to allies.

Each player's consulate gives a bonus only to that player's units. It does not give a bonus to the armies of other players who sent you reinforcements. 

You can build the Consulate once your Capitol has reached level 4.

Click on any large tile you wish to place the building on.

Next, scroll sideways through the list of available buildings until you find the Consulate icon.

Allied reinforcements hosted in your City do not increase your army upkeep cost.

You can demolish this building by following the instructions in this article.