Aletrium was a town of the Hernici[3] which, together with Veroli, Anagni and Ferentino, formed a defensive league against the Volsci and the Samnites around 550 BC.

During the reign of Theodoric the Great (5th century AD) the patrician Liberius promoted the construction of a monastic community, one of the most ancient in the West, where in 528 St. Benedict sojourned briefly.

In the following centuries it flourished economically and expanded, conquering Collepardo, Guarcino, Trivigliano, Vico and Frosinone, though often hampered by inner strife.

In the 15th century Alatri started to lose its freedom, due to the involvement of Ladislaus of Naples and Pope Martin V. After the short seigniory of Filippo Maria Visconti in 1434, the city had to accept papal suzerainty.

In the first two periods the construction is rough, while in the third the blocks are very well jointed, and the faces smoothed; they are mostly polygonal in form and are much larger (the maximum about 3 by 2 m) than those of the city wall.

Remains of a high-pressure aqueduct, which supplied the town with water and was constructed with other public buildings by L. Betilienus Varus, may still be traced.

[7] In the neighborhood are: Alatri is divided into the following rioni (quarters): Chiappitto, Pacciano, Porpuro, Valle Santa Maria, Carvarola, Capranica, Fontana Vecchia, Maddalena, Piedimonte, Madonna delle Grazie, Melegranate, Montecapraro, Vignola, Valle Carchera, Montesantangelo, Montelarena, Pezza, Allegra, Basciano, Pignano, Castello, Collefreddo, Madonna del Pianto, Montelungo, Montereo, Monte San Marino, Pezzelle, Preturo, Sant'Antimo, San Valentino, Vallecupa, Vallefredda, Valle Pantano, Vallesacco, Valle S.Matteo, Villa Magna, Cassiano, Castagneto, Fraschette, Seritico, Santa Caterina, Vicero, Aiello, Canarolo, Collelavena, Costa San Vincenzo, Maranillo, Cavariccio, Colletraiano, Imbratto, Piano, S. Colomba, Scopigliette, Cucuruzzavolo, le Grotte, Magione, Mole Santa Maria, San Pancrazio, Vallemiccina, Sant'Emidio, Canale, Prati Giuliani, Quarticciolo, Quarti di Tecchiena, Tecchiena, Campello, Mole Bisleti, Cuione, Fontana Santo Stefano, Fontana Sistiliana, Frittola, S. Manno, Arillette, Collecuttrino, Colle del Papa, Laguccio, Montelena, Quercia d'Orlando, San Mattia, Carano, Fontana Scurano, Magliano, Cellerano, Fiume, Fiura, Fontana Santa, Riano, Abbadia, Case Paolone, Fontana Sambuco, Gaudo, Intignano, Colleprata.