Because of its relatively small size, it was once considered to be no more than a "hunting lodge", but scholars now believe it originally had a curtain wall and did serve as a citadel. Frederick was responsible for the construction of many castles in Apulia, but Castel del Monte's geometric design was unique. The fortress is an octagonal prism with an octagonal tower at each corner. The towers were originally some 5 m higher than now, and they should perhaps include a third floor. Both floors have eight rooms and an eight-sided courtyard occupies the castle's centre. Each of the main rooms has vaulted ceilings. Three of the corner towers contain staircases. The castle has two entrances, an unobtrusive service entrance and an ornate main entrance. Frederick's main entrance featured elements from classical design, and may have been influenced by Frederick's interest in Greco-Roman architecture.