Fort St. Sebastian was originally constructed and named by the Portuguese c.1520-26.

However, its first appearance on a map was in the context of Di Castaldi’s Venetian map of 1564.

In putting up the fort, the Portuguese hoped to stop English ships from interfering in their trade in the Shama area.

According to a Portuguese chart of 1630, the Portuguese fort had a bastion, two single-storeyed buildings with pitched roofs and a two-storeyed tower.

At the time the Dutch took over the fort it was in a state of ruin.

While retaining its name, they carried out repairs in 1640-42 and added substantially to the previous structural form.