Guerites, or echaugettes, were small watchtowers erected at prominent points with good views, say at the angle of a bastion. It was protection from the weather for a sentry and would be taken down if the fortress was put under siege, as they were very good artillery markers for the enemy. Sometimes, the guerites were constructed in wood, such as at Neuf Brisach.
Quentin Hughes, in "Military Architecture", gives the following definition.
"ECHAUGETTE a stone sentry box cantilevered from the angle of a bastion."
Mr. Galliano, a fantastic photograph - thank you.