The construction by Claudius

The oldest lighthouse (or beacon, without a fire) in the Tiber delta that we know of must be Tor Boacciana, a tower still standing near the Ponte della Scafa ("Bridge of the Ferry"). It is located at the spot where the Tiber once reached the sea. Brick stamps point to a date around 112 AD. It received its name from the Bovazzani-family, who owned the land near the tower in the twelfth century. The tower was rebuilt by pope Martin V in 1420.

Tor Boacciana.
Photo: Wikimedia, MM.

The story of the lighthouse of Portus begins during the reign of Caligula. Pliny the Elder, when discussing the transport of two obelisks to Rome, says:

Divus Claudius aliquot per annos adservatam, qua C. Caesar inportaverat, omnibus quae umquam in mari visa sunt mirabiliorem, in ipsa turribus Puteolis e pulvere exaedificatis, perductam Ostiam portus gratia mersit.	The ship with which Caligula had brought in the other obelisk, after having been preserved for some years [in Puteoli], the most wonderful construction ever seen upon the seas, was brought to Ostia by the deified Claudius and sunk for the construction of the harbour, after towers of Puteolan ashes had been built upon it.
Abies admirationis praecipuae visa est in nave, quae ex Aegypto Gai principis iussu obeliscum in Vaticano circo statutum quattuorque truncos lapidis eiusdem ad sustinendum eum adduxit. Qua nave nihil admirabilius visum in mari certum est. CXX M. modium lentis pro saburra ei fuere. Longitudo spatium obtinuit magna ex parte Ostiensis portus latere laevo. Ibi namque demersa est Claudio principe cum tribus molibus turrium altitudine in ea exaedificatis obiter Puteolano pulvere advectisque.	An especially wonderful fir was seen on the ship which brought from Egypt, at the order of the Emperor Caligula, the obelisk erected in the Vatican Circus and four shafts of the same stone to serve as its base. It is certain that nothing more wonderful than this ship has ever been seen on the sea. It carried 120,000 modii of lentils for ballast. Its length took up a large part of the left side of the harbour of Ostia, for under the Emperor Claudius it was sunk there with three moles as high as towers erected upon it, that had been made of Puteolan ashes for the purpose, and conveyed to the place.
Pliny the Elder, Natural History 36.14.70 and 16,76,201-2.

The obelisk transported by Caligula, now on Saint Peter's square.
Photo: Wikimedia, Jean-Pol Grandmont.

Suetonius is more specific about the use of the ship: the lighthouse was placed on top of it.

Portum Ostiae exstruxit circumducto dextra sinistraque brachio et ad introitum profundo iam solo mole obiecta; quam quo stabilius fundaret, navem ante demersit, qua magnus obeliscus ex Aegypto fuerat advectus, congestisque pilis superposuit altissimam turrem in exemplum Alexandrini Phari, ut ad nocturnos ignes cursum navigia dirigerent.

Claudius constructed the harbour at Ostia by building curving breakwaters on the right and left, while before the entrance he placed a mole in deep water. To give this mole a firmer foundation, he first sank the ship in which the great obelisk had been brought from Egypt, and then securing it by piles, built upon it a very lofty tower after the model of the Pharos at Alexandria, to be lighted at night and guide the course of ships.

Suetonius, Lives of the Twelve Caesars, Claudius 20.

Pliny the Elder furthermore mentions the lighthouses of Alexandria and Portus in the same breath:

Magnificatur et alia turris a rege facta in insula Pharo portum optinente Alexandriae, quam constitisse DCCC talentis tradunt, magno animo, ne quid omittamus, Ptolemaei regis, quo in ea permiserit Sostrati Cnidi architecti structura ipsa nomen inscribi. Usus eius nocturno navium cursu ignes ostendere ad praenuntianda vada portusque introitum, quales iam compluribus locis flagrant, sicut Ostiae ac Ravennae. Periculum in continuatione ignium, ne sidus existimeretur, quoniam e longinquo similis flammarum aspectus est.

Another towering structure built by a king is also extolled, namely the one that stands on Pharos, the island that commands the harbour at Alexandria. The tower is said to have cost 800 talents. We should not fail to mention the generous spirit shown by King Ptolemy, whereby he allowed the name of the architect, Sostratus of Cnidos, to be inscribed on the very fabric of the building. It serves, in connection with the movements of ships at night, to show a beacon so as to give warning of shoals and indicate the entrance to the harbour. Similar beacons now burn brightly in several places, for instance at Ostia and Ravenna. The danger lies in the uninterrupted burning of the beacon, in case it should be mistaken for a star, the appearance of the fire from a distance being similar.

Pliny the Elder, Natural History 36.18.83.

Cassius Dio says about the harbour and the lighthouse:

In the first place, Claudius excavated a very considerable tract of land, built retaining walls on every side of the excavation, and then let the sea into it. Secondly, in the sea itself he constructed huge moles on both sides of the entrance and thus enclosed a large body of water, in the midst of which he reared an island and placed on it a tower with a beacon light.

Cassius Dio, Roman History 60, 11.

During the reign of Vespasianus, Valerius Flaccus mentions the lighthouse in his epic poem Argonautica:

Non ita Tyrrhenus stupet Ioniusque magister qui iam te, Tiberine, tuens clarumque serena arce pharon praeceps subito nusquam ostia, nusquam Ausoniam videt et seavas accedere Syrtes.	Not so thunderstruck stands the Ionian or Tyrrhenian skipper, when, as he gazes towards Tiber and the lighthouse clearly sighted 'neath a summer sky, suddenly driven headlong he sees nowhere the river mouth, nowhere Ausonia [Italy], but the fierce Syrtes [shoals of North Africa] drawing nigh.
Valerius Flaccus, Argonautica VII, 83-86.

Juvenalis mentions the lighthouse, the tremendous size of the basin, and sailors arriving "there" and shaving their head. The shaving has been interpreted as a vow, a promise made in the face of imminent shipwreck. Michele Ronnick argues that "there" is a reference to a temple of Isis, because by shaving their head the sailors act like followers of Isis.

Tandem intrat positas inclusa per aequora moles Tyrrhenamque pharon porrectaque bracchia rursum quae pelago occurrunt medio longeque relinquunt Italiam (non sic igitur mirabere portus quos natura dedit; sed trunca puppe magister interiora petit Baianae pervia cumbae tuti stagna sinus, gaudent ibi vertice raso garrula securi narrare pericula nautae.

And now at length the ship comes within the moles built out to enclose the sea. She passes the Tyrrhenian lighthouse, and those arms which stretch out and meet again in mid-Ocean, leaving Italy far behind - a port more wondrous far than those of Nature's making. Then the skipper, with his crippled ship, makes for the still waters of the inner basin in which any Baian shallop may ride in safety. There the sailors shave their heads and delight, in garrulous ease, to tell the story of their perils.

Juvenalis, Satires 12, 75-82.

The work on the harbour of Portus began in 42 AD. The lighthouse was apparently built in imitation of the Pharos at Alexandria. It too could be called Pharos, as could other lighthouses: "lighthouses (φρυκτωριοι) along the coast which by the light of their fires bring to safety ships in distress at night; the common name for such a lighthouse is Pharos" (Herodianus IV,2,8).