This small island, there are about two hundred meters from the coast, south of the mouth of the river Minho, was originally used as a place of worship. In the Christian era, it stood a small chapel under the invocation of Our Lady of Insua.
In 1392, Franciscan friars of Galicia under the direction of Fray Diego Arias, built an monastery in the insula. Will date this period the first fortification in place, with the protection function of that river bar and religious, built by determination of John I of Portugal.
In 1471 took place the convent redevelopment, being built new cells and promoted improvements in its chapel.
Later, Manuel I of Portugal (1495-1521), when passing on a pilgrimage to Santiago de Compostela (1502), has determined the implementation of new works of renovation and expansion of this defense, completed in 1512. Similarly, in the context the Philippine Dynasty, Philip I of Portugal (1580-1598) promoted her new intervention, it increased the effectiveness of artillery, urgent need to tackle the British and French pirates attacks. None of these structures come to us.
The current structure dates back to the War of Restoration context (1640-1668), under the reign of John IV of Portugal (1640-1656). Was performed between 1649 and 1652, under the orders of then Minho Arms Governor, D. Diogo Nogueira Lima.
Throughout the eighteenth century the premises of the convent and the strong were repaired on several occasions, highlighting, in 1717 a donation John V King of Portugal for the building of the church, including the dome and in 1767 the construction of new cells, the chapter house and an altarpiece. Between 1793 and 1795 the brothers left the convent to undertake repair works. In the same period in 1765 fortification found itself manned by 10 men and seven bronze artillery pieces.
During the Peninsular War the insula was occupied by Spanish and French troops. Later, with the extinction of the religious orders (1834) the fort was abandoned by the religious community, getting stocked exclusively by the Portuguese Army.