This is the great city of Tenochtitlan, constructed on an island in Lake Texcoco. This map is a representation more than a roadmap. The Spaniards were especially careful to note the presence of the large temples. You can also see the canoes used in combat. The three white lines are the three large causeways that connected the city to the mainland. Drawbridges were constructed in the causeways, which could be pulled up to prevent people from getting in or out of the city. The bridges were up on the night the Spaniards tried to escape, so Cortés constructed a portable bridge to allow them to leave, which worked until the Mexicas captured the bridge. To overcome such difficulties in the reconquest of Tenochtitlan, Cortés constructed thirteen large brigantines, or boats, which would allow the Spaniards to take control of the lake, while they besieged the city and deprived the Mexicas of food and water. You can see a brigantine at the bottom of the map.