Bunkers for the ammunition of the cannons installed in the 19th century in the fortress of Blaye (Gironde, France). These cannons were installed on circular platforms. We see a part of one of these platforms on the right of the photo. These cannons can struck all the width of the estuary and made obsolete the cannons on the islands in the middle of the estuary.
<a href="//commons.wikimedia.org/wiki/User:Tangopaso" title="User:Tangopaso">Tangopaso</a>
<span class="int-own-work">Self-photographed</span>