Missions are scattered all over Texas, with lots of buildings still standing. Really fun places to visit and learn.
Though it was indeed used as a fort during the Texan war for independence. Basically it was taken by Travis and his men, with a force of around 200 men. Then the Mexican general Santa Anna came calling with his crew around 2000 men. Called for surrender, but the Texans refused. The Texans held for... 13? days. When I was a kid the story was they killed 7 for every one they lost, but I think that has long since been toned down to like 3 or 4. Still pretty damn impressive imo. After that victory, Santa Anna was crushed by Sam Houston who arrived too late to save the men at the Alamo. "Remember the Alamo!" became the battle cry for the whole of the independence forces.
Parts of the main building of the Alamo are still standing and you can visit it in San Antonio. It's actually pretty neat, but so are most historical sites imo. There is a list of names when you go in there starting with Travis, Davy Crockett, and Jim Bowie (Bowie knives are named after him). My grandmother claims a shared lineage with the fourth name on the wall: James Butler Bonham, and I do believe in recent years she has obtained proof via public record and hunting tombstones all over TX.
As for your quip about "invaders from the north," that is only very partially true. Some of the men I know where from TN come to help fight, but most were local german and mexican men who wanted independence from Mexico. They won obviously and that's how The Republic of Texas was formed and later joined with the USA.
Sportsmanship killed CF