The fortress stands atop a rocky massif that separates the courses of the Abión and Ucero rivers and is separated by a pass from the moors bordering the Duero valleys to the south.

The town of Osma is first documented in 912, and although one might assume it was a resettlement of the ruins of the Roman city of Uxama, there is evidence of structures on the hill where the castle now stands that suggest this early settlement was located there.

Around 933, when the first defeat of the Caliphate’s troops occurred at Osma, or in 934 when Ramiro II of León and Count Fernán González took refuge in the castle (it is unclear whether this was the castle in Osma or the one in Gormaz), the Leonese must have been constructing most of the fortress we see today. The strategic importance of Osma Castle increased significantly following the Muslim defeat in 939, which followed the Battle of Simancas, south of Gormaz. The Caliphate responded in the following years by recapturing Gormaz and fortifying it. Thus, for fifty years (between 934 and 989), Osma Castle served as the main defense of the Kingdom of León’s borders along this stretch of the Duero River and was undoubtedly the Christian castle most exposed to attack by the formidable Muslim enemy.

Ramiro II of León’s construction was attached to the earlier towers and was built of thick masonry featuring an abundance of Roman stones and superb ashlars carved with stonemason’s marks at the corners, doors, and arrowslits. The upper castle had four towers (the two pre-existing ones and the two new ones, one of which was pentagonal and hollow) and two gates—one to the north and one to the south—which allowed access down to the Abión and Ucero rivers.

Fifty years of resistance against the Caliphate and the formidable fortress of Gormaz is a long time, but in the end, Almanzor was Almanzor, and Osma fell into his hands in 989. In 994, San Esteban and Clunia fell, and the following year, the Castilian count Garci Fernández was beheaded at Alcozar.

The famous general from Algeciras repopulated the area with Arabs and refortified the castle of Osma; part of the outer cladding of the tower facing Uxama may be his work. The castle remained in the hands of the Caliphate until 1011, when it passed back into Christian hands.