The Consulate allows you to host and send reinforcements. It also grants bonus health and strength to reinforcements that you send to allies. You can build the Consulate once your Capitol has reached level 4.

Select any large tile in your City.

   
 
Scroll down to find the Consulate and confirm construction.
 
 
Tip
 
Allied reinforcements hosted in your City do not increase your army upkeep cost.