How did just a handful of Spaniards topple the mighty Aztec Empire?

Answer 1

The most notable achievement of Cortés' career was the invasion of Mexico and conquest of the Aztec empire. In 1518 Velázquez put him in command of an expedition to explore and secure the interior of Mexico for colonization. At the last minute, due to the old gripe between Velázquez and Cortés, he changed his mind and revoked his charter. Cortés ignored the orders and went ahead any way in an act of open mutiny.

On the way to Tenochtitlan, Cortés made alliances with native American polities such as the Nahuas of Tlaxcala and the Totonacs of Cempoala. By the time he arrived in Tenochtitlan the Spaniards had a large army. At first, the Aztecs received Cortés' forces peacefully in Tenochtitlan where they stayed as guests at the palace of Moctezuma II. The balance turned and Moctezuma became a prisoner in his own house while the Spaniards began surveying his empire. Moctezuma became a prisoner because he gave lavish gifts as in gold to the Spaniards which enticed them to plunder vast amounts of gold. When the Aztecs perceived the weakened state of their ruler they staged a rebellion in order to kick out the Spaniards and their native American allies from the city. In the Noche Triste the intruders were forced to flee over the causeway to the mainland and back to Tlaxcala. On the way they suffered great losses. But the Spaniards and their allies prevailed: with reinforcments arriving from Cuba, Cortés began a policy of attrition towards the island city of Tenochtitlan cutting off supplies and subduing the Aztecs' allied cities thus changing the balance. The decisive battle in this campaign was the siege of Tenochtitlan. Cortés' victory over the Aztecs at Tenochtitlan enabled the eventual Spanish conquest of Mexico.
http://en.wikipedia.org/wiki/Hern%C3%A1n_Cort%C3%A9s#The_Invasion_of_Mexico

Answer 2

they had horses and chariots and superior weapons.
These things were not known to the new world.