The Summa Theologiae of St. Thomas Aquinas (+1274AD) -- his summary of theology -- is arguably the greatest work of Catholic theology and philosophy. It is also a splendid example of a scholastic-style treatise and the kind of intellectual product we have from the medieval university. He began the work in 1265AD, but didn't not finish before his death. The supplement section was completed by some associates of his using his notes and other works. The Summa Theologiae is available here in English translation:
St. Thomas Aquinas, the Angelic Doctor, a Dominican from southern Italy, studied in Cologne under St. Albert the Great, taught at the University of Paris, and spent a fair amount of time teaching and preaching in Italy, as well. He is justy famous for composing the Mass and hymns for the new Feast of Corpus Christi established by Pope Urban IV (Pontiff from, 1261-1264AD). Urban had been Patriarch of Jerusalem in the Crusader states, another interesting story all its own, for another day...
St. Thomas Aquinas, painted by Fra Bartolomeo (+1517AD)