Archaeological excavations have traced human presence on the castle hill to at least the 8th century BC, when Celtic tribes used the commanding height as a lookout over the Tagus valley. Phoenicians came next, followed by Greeks and Carthaginians, each recognizing the strategic value of a position that controlled access to the river and the Atlantic beyond it. The Romans erected the first formal fortification around 48 BC, when Lisbon was classified as a Roman municipality. After Rome fell, the hill changed hands to the Suebi, then the Visigoths. In the 10th century, Berber forces built the walls known as the Cerca Moura, the Moorish Encirclement, and for two hundred years the fortress was the center of Islamic Lisbon.

The castle's Christian history began with the Siege of Lisbon in 1147, when King Afonso Henriques, aided by Northern European crusaders on their way to the Holy Land, captured the city from its Moorish rulers. The castle became the alcacova, the fortified royal residence, and when Lisbon replaced Coimbra as the capital of the Kingdom of Portugal in 1255, it became the seat of power for Afonso III. The castle and its walls withstood multiple Castilian assaults in the 14th century. It was King John I, who had married the English princess Philippa of Lancaster, who dedicated the fortress to Saint George in the late 1300s, giving the castle the name it still carries. In 1494, the German humanist Hieronymus Munzer visited and was particularly impressed by the lions kept in the castle grounds, calling them the most beautiful wild beasts he had ever seen.

The castle's roughly square plan, with ten towers and walls extending far beyond the citadel into the surrounding neighborhoods, reflects centuries of incremental construction. The main entrance, a 19th-century gate bearing the coat of arms of Portugal and the name of Queen Maria II, leads to the Praca d'Armas, decorated with old cannons and a bronze statue of Afonso Henriques copied from the 19th-century original by sculptor Antonio Soares dos Reis at Guimaraes Castle. The medieval castelejo occupies the highest point, its walls still walkable, its towers offering views across the entire city and the Tagus. The Tower of Ulysses, which once housed the Torre do Tombo national archive, now contains a camera obscura installed in 1998 that projects a live, 360-degree image of Lisbon onto a white dish in a darkened room.

Beyond the inner fortress, a barbican, a lower outer wall, protects the southern and eastern flanks from siege engines. The northern and western sides need no such defense: the hillside drops steeply enough that attackers would exhaust themselves on the climb before reaching the foundations. A dry moat, once filled, encircles part of the perimeter, and a stone bridge spans it at the main entrance. On the western slope, a long curtain wall extends downhill to the Torre de Sao Lourenco, which once guarded a gate in the Cerca Fernandina, the later medieval wall that enclosed a much larger portion of the city. The ruins of the royal palace near the main square hold the Olissiponia exhibit, a multimedia history of Lisbon presented in the same rooms where kings once governed.