Why Did the Inca Siege of Cuzco Fail?

Si Sheppard says the Inca siege of Cuzco failed because narrow streets kept a huge army from hitting the remaining conquistadors all at once.

Surviving conquistadors fell back to two heavily fortified buildings with narrow entrances. The streets of Cuzco are narrow, so overwhelming numbers could move only down the avenues, and small numbers of conquistadors plus growing native allies blocked them in metal armor. When the pressure eased, horses went out through the wider plazas and the surrounding countryside to hit isolated detachments. Manco had rallied the Inca to finish the Spanish for good, but as the siege dragged on, men slipped away at night to switch sides or go home. The army was huge and diverse, and many recruits were peoples the Inca had subjugated, with no institutional loyalty. They needed to get home to their farms for the harvest before winter. Warfare in pre-Columbian America, and in Europe for much of its history, followed the yearly cycle of planting and harvesting. If the men were not on the farms, and not enough crops were set aside, the winter and the following year would be very hard.

Huge numbers still could not be applied all at once. If the entire conquistador army had been caught in a mountain pass, the Inca may have won a set-piece battle. Horses cannot charge steep slopes, and blocking both ends with boulders and trees would pin the infantry, while metal armor would not stop a two-ton boulder.