At each tile, it is necessary to build a house. Houses are single-floor, two-floors, three-floors, etc.
Single-floor house can be built on any tile. Two-floors house can only be built on a tile adjacent to built a one-floor house. A three-floors house can be built only on the tile, in which the neighbors are one-and two-floors houses. And so on.
Objective - to build city as much as possible floors.