The subject of this painting is a Dutch naval attack on a fortress by the sea which is defended by Spanish ships and armed soldiers. The fort stands on raised ground, on the left, behind some low tree-topped dunes. A long line of armed men can be seen at the foot of the fortifications. The Dutch men-of-war are sailing in from the right under a fair sky but waves lash the sand. In the foreground there is a galley flying the Burgundian flag, which was carried by all ships of the Spanish-occupied Netherlands, and a striped awning on the stern. The Spanish man-of-war beyond it flies a flag showing the Virgin Mary upon a crescent moon. It is shown in stern and starboard view, full of armed men, setting off to confront the invaders. The stern is ornately carved. Ahead of her another galley flying the Spanish flag prepares to engage with the four Dutch ships who are advancing from the right. These are flying the Dutch flag with the one nearest to the Spanish man-of-war shown firing. Separated from the action, in the immediate foreground on the left, a number of musketeers observe the scene.

The painting shows an imaginary landscape which, through his introduction of the fort and the Mediterranean galleys, Willaerts’ audiences would have understood to represent a foreign shore. It has been suggested that this may be a place in either the Far East or Brazil, where the Dutch and the Spanish had colonial interests. Although, fighting between Spain and the rebellious United Provinces had virtually ceased in the Netherlands by 1609, they remained in a state of war until the Treaty of Münster was signed in 1648. Two pen-and-wash drawings which Willaerts contributed to the Album Amicorum Arnold van Buchell, now at Leiden University, seem to relate directly to the present painting. These show a Spanish galley in front of a fort on the left and a Dutch man-of-war sailing in from the right, firing its guns. A slightly larger painting in the Musées Royaux in Brussels which, Spicer discusses as 'Spanish Ships Defending a Portuguese(?) Fort from Dutch Attack', compares with the present picture in its rendering of the Spanish and Dutch ships and the large figures, in the foreground, in their carefully observed costumes and stylized poses. Spicer links the geographical setting of the painting in Brussels to increased Dutch naval attention along the Portuguese coast after 1621. This might, therefore, be an alternative setting for this painting.

Despite the emergence of a more atmospheric style of Dutch marine painting elsewhere, especially in Haarlem, Willaerts’ style remained recognizably colourful throughout his life. In this particular work, the rendering of the figures is reminiscent of Jacob de Gheyn’s drawings of musketeers, 1607. Willaerts had a monopoly on seascape painting in Utrecht, engaging with its intellectual elite and gaining prestigious commissions from the City and State of Utrecht, which could have included this picture. Adam Willaerts was born in London in 1577. He is recorded in Leiden in 1585, in Amsterdam in 1589 and settled in Utrecht in 1597. In Utrecht he was involved in the founding of the local Guild of St Luke and remained closely associated with its management, frequently serving as its deacon. He had a number of apprentices and three of his own children, including Abraham, became painters as well. He occasionally collaborated with the still-life artist, Willem Ormea. Willaerts, renowned for his lively coastal scenes and his sea storms, presented a painting of a 'Storm at Sea to the Hiobsgasthuis' (Hospital of Job) in Utrecht, in 1628. He was buried in Utrecht in 1664. The painting is signed lower left '16 A22/Willaerts' immediately below the sword of man dressed in grey.