On 10 November 1892 Jorge O´Neill applied, in Cascais Town Hall, for a permit for the construction of a house near Boca do Inferno, in the locality of Santa Marta and including the site where there is pigeon shooting.

The construction of the Tower of S. Sebastião, now the Museum Counts of Castro Guimarães, began around 1900.

According to popular lore, the project, by the painter Francisco Vilaça, was inspired by the sets of an opera the Italian choreographer Luigi Manini had put together near the bay of Santa Marta.

The building sports an abundance of styles, from the feudal castle to the Moorish, manueline and renaissance influences, using several materials, raging from stone to plaster, mortar and ceramic coating.

In 1910 was sold to the count Manuel de Castro Guimarães. As there were no heirs, he bequeathed his magnificent house, as well as his collections and books and delightful park of glazed tile recesses with all the furniture, objects of art, books and silver therein to the municipality of Cascais, to become a small municipal museum and public library.

Opening on July 12, 1931 it was named Library Museum of the Counts of Castro Guimarães, at the specific request of its founder, so that it would include his wife, Ana de Castro Guimarães.

In adapting it to a museum it was intended to preserve the ambience of a house where people lived in, benefiting the interior architecture and the decorative motifs, harmoniously mixing the art collections bequeathed by the benefactor and those acquired later.

In the surrounding park you can find two figurative tile panels from the 18th Century, attributed to Bartolomeu Antunes.

In the chapel of S. Sebastião, (17th century) formely the property of the citadel and today annexed to the museum, you can observe the façade with remarkable examples of Portuguese tiles, the nave and chancel, from the time of the foundation. The tiles representing the life of the saint, date from the first half of 18th century.