A close friend of President Antonio López de Santa Anna, Castrillón frequently advocated merciful treatment for captured Texian soldiers.
He joined Santa Anna on the 1836 invasion of Texas, which first journeyed to San Antonio de Bexar, and besieged the small Texan force garrisoned at the Alamo.
[2] On March 6, 1836, during the final assault of the Alamo, Castrillón took command of the Toluca Battalion after Colonel Francisco Duque was wounded.
[5] Weeks later, during the Goliad Massacre, Fernández Castrillón also protested – in vain – the execution of nearly 400 Texian prisoners, including their leader, James Fannin.
Castrillón saw no further fighting until April 21, 1836, when Texas General Sam Houston launched a surprise attack on Mexican forces at the Battle of San Jacinto.
As Texian forces jumped the makeshift barricades surrounding the Mexican army encampment, Castrillón, Santa Anna, and Colonel Juan Almonte all began shouting orders, some contradictory, in the hopes of rallying their troops to mount a defense.