The Alamo is a Spanish mission, built in 1718 in what became the city of San Antonio. In 1836, the citizens of the then Mexican state of Coahuila y Tejas tired of the dictatorial rule of Mexican president Antonio Lopez de Santa Anna and began a fight for independence, known as the Texas Revolution.
In March of that year, battle between an estimated 180-200 members of a ragtag militia (known as "Texians") and 4,000 Mexican troops took place at the Alamo site. The result of the battle was a Mexican rout; all Texian defenders were killed. However, it is considered a turning point in the Texas Revolution in that it tied up Mexican forces long enough to allow the supreme commander of the Texian army, General Sam Houston, sufficient time to organize his forces; Santa Anna was defeated the following month and Texas gained its independence.
What is commonly referred to as "The Alamo," at the time of the famous battle for Texas Independence which was fought there, was a crumbling, abandoned mission which had been built by Catholic Missionaries about a hundred years earlier, while Spain was exploring and colonizing what is now the southern US and Mexico.
At the time of its construction, the mission consisted of the sanctuary, which is the familiar structure we see in most photographs today, quarters for the monks and "friendly," "converted" Indians, grain storage facilities, water access, barracks for quartering soldiers there for security, and a stone perimeter wall to prevent or reduce the devastating results of attacks by "hostile" Indians.
By 1836, due to the long departed threat from hostile Indians, the mission had been long since abandoned, and without the required continuous Maintenance, the mortar holding the stone masonry together was failing, allowing many structures to collapse or at least partially collapse. The "Texian" rebels, under command of Col. William B. Travis made hasty makeshift repairs by simply "stacking" the collapsed stone sections of the perimeter walls, and barricading those sections where the stone had, over the years, been "salvaged" for reuse by neighbors in the surrounding area.
Thus, Travis' command converted the crumbling, long abandoned mission complex into a crude makeshift fortress, which as we all know from history, was totally inadequate to withstand the assaults of more than five thousand troops with heavy artillery, under the command of General Santa Anna, Emperor of Mexico.
The significance of the Alamo and the battle fought there is that Travis' troops managed to delay Santa Anna and his troops long enough for Texian General Sam Houston to gather and prepare for future battle with the Mexican Army.