Casa Sommer was built at the end of the nineteenth century for the Sommer family, German industrialists who made their fortune in Lisbon. The house was restored and reopened in 2017 as the home of the Cascais Municipal Historical Archive — and as such houses fascinating documents on local history, royal-era photographs, and the maps from which the modern town was planned.
The Sommer Library, on the upper floor, holds about 25,000 volumes covering Portuguese history, art, and travel writing — including a number of nineteenth-century editions on the discovery of Cascais by the royal family. The building itself, with its ornamented facade and well-preserved interiors, is worth looking into even if archives aren't your thing.