This is the great city of Tenochtitlan, constructed on an island in Lake Texcoco. The city is the large white spot in the middle of the map. You can see houses on the edge of the island. 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.