The Texian garrison of the Alamo consisted of about 185 men, besides
women, children, and slaves. This is a major weakness as they were greatly outnumbered; The Mexican army consisted of about 7,000
men. The Texians were thus outnumbered about 37 to 1.
There are other several weaknesses. First, it was an exposed position.
Second, its small physical size.
The alamo also had adobe walls.
The average siege
with firearms and cannon requires at least a 20 to 1 numerical
superiority in manpower and firepower in order to succeed. The Texians
only lost once they ran out of ammunition.