Frederick II Tower, Medieval tower in Enna, Italy.

Frederick II Tower is an octagonal stone tower in Enna, built in the first half of the 13th century and attributed to the architect Riccardo da Lentini. Each floor has an umbrella vault, and the ground level has almost no windows. The two rooms inside measure no more than four metres per side.

The octagonal tower is attributed to Emperor Frederick II, who had it built by architect Riccardo da Lentini in the first half of the 13th century, a thesis supported by Giuseppe Agnello. In August 1233 Frederick wrote to Pope Gregory IX from Castrogiovanni, as Enna was then called. Some scholars instead name Frederick III of Aragon, crowned king of Sicily in 1296, but academia has abandoned this position. In the city cadastre it is recorded as the Torre di Alcontres.

Local records in Enna's state archive list the building as the "Torre di Alcontres", a name that survives in the city's cadastral registers. The octagonal plan, with umbrella vaults on each floor, follows a north European prismatic keep model rather than a Sicilian palace layout. Its two rooms, roughly four metres on a side, are mostly windowless at ground level.

The tower stands on Enna's highest point, near the Castle of Lombardy, and is reached on foot along the city's steep streets; the walk from the centre involves a sustained climb, so sturdy shoes and extra time help. The octagonal structure has two small rooms per floor and little ground-level lighting, so a visit is short and best timed for daylight.

The tower stands at considerable elevation and gave guards a sweeping view across central Sicily, making it a crucial observation post. From this vantage point, movements could be tracked over many kilometers away.