Reading the "COMMENTARII" of Pio II, we know everything about the work executed to build the Rocca. The adjacent roman amphitheater of Bleso was levelled not giving in this way any shelter to the enemies, the massive wall of the medieval city town-walls was raised in the north part adding a door, defended from a bastion, that is situated where today resides the restaurant "Taverna of the Rocca" .
The Rocca is composed by four towers of various dimensions joined together from high massive walls crowned with hanging bows inspired by the medieval architecture. Spaces needed to introduce the fire mouths are visible in some parts. The two greater towers (which height is of m.36,50 and m. 25,50) are situated towards outside in order to get the best defence from an eventual external attack.
The diameter of the greater one is of m.13,20 and contains
6 overlapped rooms, while the diameter of the other is of
m.10 and this tower has 5 overlapping rooms. On the contrary,
the two smaller towers are opened towards the inner courtyard
in order to control the inside of the fortress and the city.
These two towers are similar but they have a different height:
m.18,50 vs m. 15,50; anyway both of them contain 3 overlapping
rooms. The thickness of the walls is very large because, at
the time of the building, the powder was asserting itself:
in the greater tower, indeed, the diameter is of m.3,90, in
the median one m.2,90 and in the other two m. 2.
The entrance to the castle , controlled with a drawbridge, is located on the north side. On the portal you can read a Latin text : "GRATE BONIS INVISA MALIS INIMICA SVPERBIS SVM TIBI TIBVRE ENIM SIC PIVS INSTITVIT", that sounds as: "I am here, for you, oh Tivoli, loved from the good people, hated from the bad ones, enemy for the arrogant ones: Pio II wanted all this".
During the 1700's, the Rocca was occupied by the French and the Austrian troops and then it was used as prison and barracks. In the period of Napoleon, the building was definetely turned into a prison adding a factory near to the northern wall. Then the Rocca continued to be used as a prison until 1960.