After the Spanish and their allies regrouped in Tlaxcala, Cortés decided to attack Iztapalapa before besieging the Aztec capital of Tenochtitlan, in part to secure supplies. The city of Iztapalapa had about 10,000 people with about two-thirds of its structures built over water. Eight thousand Aztecs defended the city on land and on water. Cortés and his indigenous allied burned, massacred and destroyed the city completely. But after the battle, when the Spanish let down their guard, Iztapalapa opened water channels to flood the city and drown the invaders. Most of the Spanish survived, but many of their Indian allies did not and were drowned. The Spanish lost all of their gunpowder.