Read this: Gallaeci lived in castros. These were usually annular forts, with one or more concentric earthen or stony walls, with a trench in front of each one. They were frequently located at hills, or in seashore cliffs and peninsulas. Some well known castros can be found, in the seashore, at Fazouro, Santa Tegra, Barona and O Neixon, and inland at San Cibrao de Las, Borneiro, Castromao, and Viladonga. Some other distinctive features, such as temples, baths, reservoirs, warrior statues and decorative carvings have been found associated to this culture, together with rich gold and metalworking traditions.

Where were these forts commonly built?
What is the answer? (If it cannot be answered, return "unanswerable")
hills