Why were the alamo defenders inside the fort?

Houston sent Jim Bowie to San Antonio: his orders were to destroy the Alamo and return with all of the men and artillery stationed there. Once he saw the fort's defenses, Bowie decided to ignore Houston's orders, having become convinced of the need to defend the city.

Seeing the massive Mexican army on their doorstep, the Texan defenders hastily retreated to the well-fortified Alamo. During the first couple of days, however, Santa Anna made no attempt to seal the exits from the Alamo and the town: the defenders could very easily have slipped away in the night if they had so desired.

But they remained, trusting their defenses and their skill with their lethal long rifles. In the end, it would not be enough.

The Defenders Died Believing Reinforcements Were on the Way