This is the fort’s sea entrance. At one time, Peniche was an island until the causeway finally filled in sufficiently to be a permanent connection to the mainland.
This fort was in use up until 1974. In the early 20th century, the dictator Salazar used it as a prison for “trouble-makers” to his regime. After that, it was used as a refugee camp to shelter part of the influx of émigrés from the previous Portuguese colonies after they were freed.