[5][7] In the late 15th century, King John II had designed a defence system for the mouth of the Tagus that depended on the fortresses of Cascais and São Sebastião (or Torre Velha) in Caparica on the south side of the river.

The project was started on a basaltic rock outcrop a short distance from the riverbank, using some of the stone being collected to build the Monastery of Santa Maria de Belém.

The tower was designed by military architect Francisco de Arruda,[11] named "Master of the works of the Belém stronghold" by King Manuel,[12] and in 1516 he began receiving 763 blocks and 504 stones for its construction, delivered by Diogo Rodrigues, treasurer for the project.

A rectangular two-storey space was constructed over the bastion, giving the tower the visual profile that it has retained to the present, with sculpted crosses of the Order of Christ and domed turrets.

In 1589, Philip I of Portugal ordered Italian engineer Friar João Vicenzio Casale to build a well-defended fort to be constructed in place of the "useless castle of São Vicente".

Similarly, a reference to the year 1655 was inscribed on a plaque placed on the northern wall of the cloister, which certified the tower's function as a customs control point and for navigation along the Tagus; vessels were obliged to pay a tax as they entered the harbour, which was imposed incrementally.

Between 1780 and 1782, under the reign of Maria I of Portugal, General Guilherme de Valleré constructed the Fort of Bom Sucesso, whose battery was connected by a western corridor wall to the tower.

During the reign of Maria II, Almeida Garrett protested the site's degradation and under the persuasion of the Duke of Terceira, renovations were begun by military engineer António de Azevedo e Cunha.

[10] He demolished the Philippine barracks and extended revivalist elements in 1845–46 (such as the armoured merlons, the balustrade of the veranda along the southern façade, the laced fascia in the cloister and the niche with an image of the Virgin and Child).

In 1865–67 a beacon was installed on the southeast terrace of the building and a telegraph service was started, while nearby a gas factory was built, producing smoke that prompted many protests.

[10] In 1983 the site hosted the 17th European Exhibition on Art, Science and Culture, and various projects involving the building were undertaken, among them covering the cloister with a transparent plastic cupola.

The Portuguese Ministry of Culture and the Institute of Architectural Heritage indicate that the tower was constructed on a small island near the bank of the Tagus, opposite the shore of Restelo.

[4][20] This is especially apparent in its elaborate rib vaulting, crosses of the Order of Christ, armillary spheres and twisted rope, common to the nautically inspired organic Manueline style.

On the northeast angle of the structure, protected by a defensive wall with bartizans, is a drawbridge to access the bulwark, decorated in plant motifs, surmounted by the royal coat of arms and flanked by small columns, complemented with armillary spheres.

The Manueline armillary spheres appear at the tower's entrance, symbolizing Portugal's nautical explorations, and were used on King Manuel I's personal banner to represent Portuguese discoveries during his rule.

[24] The upper tier of the bastion is crowned by a small wall with bartizans in strategic places, decorated by rounded shields with the cross of the Order of Christ encircling the platform.

[25] The bartizans, cylindrical turrets (guerites) in the corners that served as watchtowers, have corbels with zoomorphic ornaments and domes covered with ridges unusual in European architecture, topped with ornate finials.

The third floor has twin windows in the northern, eastern and western façades, with balusters, interspersed by two armillary spheres and a large relief with the Royal coat of arms.

[27][28] The interior of the bastion, with a circular staircase at the north end, has two contiguous halls with vaulted ceilings supported by masonry arches, as well as four storage lockers and sanitary facilities.

[6] The first-floor interior contains the Sala do Governador (Governor's Hall), an octagonal space that opens into the cistern, while in the northeast and northwest corners are corridors that link to the bartizans.