This unique construction has a quadrangular ground plan of approximately 11.75 m on each side and a total height of 55 m, of which 34.38 m correspond to the Roman construction and 21 m to the restoration carried out in 1789 to modernise the maritime signalling system. Today the tower stands on a broad polygonal platform 32.40 m wide, which serves as its base and dates from the beginning of the 19th century.

In terms of height, the tower retains 37.58 m of the Roman structure, of which 34.38 m are hidden under the neoclassical façade and the remaining 3.20 m under the platform that was built at the beginning of the 19th century, easily visible from the excavation area.

To this structure would have been added the crowning body where the Roman rotunda was located. According to 18th century documentation, this structure measured 8 varas (ca. 6.40 m) in diameter and 5 varas (4 m) high, which would give the tower an original height of around 41.58 m. With these dimensions, the lighthouse would be one of the tallest preserved Roman buildings, surpassed by the Pont du Gard (France) at 49 m and Trajan's Column (Rome, Italy) at 38 m in height, but taller than the Column of Marcus Aurelius (Rome, Italy) at 30 m, the aqueduct of Segovia (Spain) at 28.5 m and the aqueduct of Les Ferreres (Tarragona, Spain) at 27 m.

The Tower of Hercules is currently 55 m high, of which 34 m correspond to the Roman construction and 21 m to the restoration directed in 1789 by the military architect Eustaquio Giannini, in order to provide the old lighthouse with a new lantern, to which the beacon was added in 1806. These measurements make the tower the only Roman lighthouse with a significant part of its structure still standing, even though it is hidden under a neoclassical façade.

The tower is arranged into three levels or floors, each with a different height: 9.20 m, 9.60 m and 14.35 m respectively. Each floor has a similar layout, with four independent quadrangular chambers measuring 2.70 m on each side. All of these spaces were crowned with barrel vaults, but three of them were removed when the interior staircase was built.

The dividing walls are 1.5 m thick, which is the same width as the original walls of the core of the Roman structure. Today, this thickness is greater because when the original construction was lined on the outside with a kind of façade between 1788 and 1790, the Roman structure was covered and fused to this lining. The end result was that the walls went from a width of 1.50 m to the 2.10 or 2.15 m they have today.

Lat. 43º 23' 13.3923" north latitude.

Long. 8º 24' 17,9519" west longitude.

The topographical height of the base of the tower is 57 m above sea level.

The UTM coordinates are:

X = 548.200

Y = 4.804.060

Zone = 29