The hill on which Saint George's Castle stands has played an important part in the history of Lisbon, having served as the location of fortifications occupied successively by Phoenicians, Carthaginians, Romans, and Moors, before its conquest by the Portuguese in the 1147 Siege of Lisbon. Since the 12th century, the castle has variously served as a royal palace, a military barracks, home of the Torre do Tombo National Archive, and now as a national monument and museum

In the context of the Christian Reconquista, the castle and the city of Lisbon were freed from Moorish rule in 1147 by Afonso Henriques and northern European knights in the Siege of Lisbon during the Second Crusade; this victory was the only notable success of that failed crusade.[7] According to an oft-repeated legend, the knight Martim Moniz, noticing that one of the doors to the castle was open, prevented the Moors from closing it by throwing his own body into the breach, thus allowing Christian soldiers to enter at the cost of his own life. With the taking of the castle Christian forces were able to maintain the defense of Lisbon until the end of the 12th century.

Apart from its main walls, the castle is protected, on its southern and eastern sides, by a barbican (barbacã), a low wall that prevented siege engines from approaching the main castle walls. The northern and western sides of the castle, on the other hand, were naturally protected by the steep hillside sloping downward from the castle's foundations. The castle is also partially encircled by a moat, now dry. The main entrance is fronted by a stone bridge across the moat. On the west side, there is a long curtain wall extending downhill, ending at a tower (the Torre de São Lourenço).[9]: 33–35 This tower guarded a gate in the Cerca Fernandina.

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