Mexica plots the development of the city of Tenochtitlan on an island in lake Texcoco. Players attempt to partition it into districts, place buildings, and construct canals.
Districts are formed by completely surrounding areas of the island with water and then placing a District marker. The player who founds a district scores points immediately.
Canals and Lake Texcoco act as a quick method of moving throughout the city. Players erect bridges and move from one bridge to the next, which costs 1 action point regardless of the distance. They must also erect buildings. This costs action points, the exact number being dependent upon the building's size.
In the scoring phases of the game, players score points (El Grande style) based upon their dominance in a District. In the 4 player game, players with the 1st, 2nd, and 3rd most buildings score decreasing numbers of points.
Only districts are scored in the first scoring round.
In the second scoring round at the end of the game, all land areas are scored, not just districts.