sentences:  The centre of the peninsula (the Middle Mark of Al-Andalus or "aṯ-Ṯaḡr al-Awsaṭ") became a strategic military post in the 11th century.The Muslim rulers created a defensive system of fortresses and towers all across the region with which they tried to stop the advance of the Christian kingdoms of the north.The fortress of "Mayrit" (Madrid) was built somewhere between 860 and 880 AD, as a walled precinct where a military and religious community lived, and which constituted the foundation of the city.