In 1500 Julius II decided to equip the city of a fortress to protect it from the threat of pirates, and put in charge Bramante, and then his students Giuliano Leno and Antonio da Sangallo. The keep, however, was entrusted to the great Michelangelo. The fortress was founded over a large Roman building of the imperial era. Fort Michelangelo is among the greatest of his era and has the shape of a quadrilateral, with four towers and an octagonal Keep. Everything inside ran around the moat and the walls are lined with travertine and crowned by parapets with openings for cannons and muskets. The fortress could be completely isolated from the rest of the fort so you could focus the defence. Above the old entrance, between the Keep and the west tower, one can still see the bronze pulley of the drawbridge on the doorposts of which is carved the order: "LEAVE YOUR WEAPONS". In the tower of San Sebastiano is housed an underground corridor as a secret exit from the fortress to the grounds.