Complex sentence: Perhaps the most important development for Marvão at this time was the donation, by Sancho II of Portugal in 1232, of extensive domains south of the Tagus to the Knights Hospitaller, along with the duty to fortify the frontier and help in its repopulation.
Split into simpler sentences: Perhaps the most important development for Marvão at this time was the donation of borderlands to the military orders. In 1232, Sancho II of Portugal donated extensive domains south of the Tagus to the Knights Hospitaller, along with the duty to fortify the frontier and help in its repopulation.