https://travelfrance.tips/tregastel-castle-chateau-de-costaeres/

https://www.openstreetmap.org/way/82084507 Aerial street view: https://goo.gl/maps/cZPWKLRnoxy7ZVg29

This place seems to be a private residence, not available to visit.

Château is not technically accurate: the château is actually a big neo-medieval style manor characteristic of the great summer houses of the late 19th century on the côte de granit rose (pink granite coast).

The building, a voluminous complex resulting from several extensions, is made of pink granite from the quarries of La Clarté, Perros-Guirec district. The roof is slate.

Its interior was designed with reclaimed wood from a three-masted sailing ship beached in the winter of 1896, the Maurice.

The manor was built on an islet bought at the end of the summer of 1892 by Bruno Abakanowicz (also called Bruno Abdank, who a little later - around 1896 - built the Bellevue hotel in Ploumanac'h), engineer and mathematician of Polish origin, from the customs officer René Le Brozec, a Perrosian who cultivated potatoes there and dried lichen and fish. The going rate at the time was 0.25 F per square metre. It was completed around 1896 by the engineer Lanmoniez and the Lannionnais entrepreneur Pierre Le Tensorer.