The harbour at St-Germain-sur-Ay attracted a good deal of commercial activity in the past due to its salt works and sea sand.  Even up to the end of the 19th C, coal was still regularly delivered by boat.  As a result a guard room was built in 1669 in order to keep a watch over the entrance to the port.  Once its original purpose had come to an end it became a chapel, known as "the Rosaire" or "the Grapillon" (the latter is the name of a short grass which grows locally).

The guardroom is listed by Monuments Historiques et Sites and its restoration has been undertaken since 1978 by a local conservation group.

Open for unguided visits. free entrance.