On February 25th, 1836 approximately 425 Mexican infantry under General Castrillon attacked the Alamo from the West. After a few hours fighting, the Texians forced a retreat. Later in the day, President General Antonio Lopez de Santa Anna arrived at San Antonio de Bexar, which had mostly, but not completely been deserted upon news of the approaching Mexican Army. A story once considered apocryphal but now supported by several references is that Santa Anna found Melchora Iniega Barerra and her mother in one of the houses and staged a mock-marriage in order to get some satisfaction. Melchora and her mother were sent back to Mexico City after the siege, and she later provided Santa Anna a son.
Late in the day, a Texian scouting party burned the huts and larger buildings of Pueblo de Valero, the small village located between the Mission and the river which had served as housing for those connected to the Mission. While the Mexicans were able to establish emplacements in Bexar across the river, this denied the Army the cover of buildings placed quite near the walls of the Mission-Fort.
That night, William Travis sent Juan Seguin out of the fort to carry news and a request for re-enforcements to General Sam Houston.
This map shows the layout of the village of Bexar, the river and the Mission-fort. The top is East; North on the Left.
Compare to this:
Layout of the fort; compare to the brown outline in the picture above: