Additional rock armouring is being placed at L'Ancresse wall in Guernsey.
It is to give the deteriorating World War 2 anti-tank structure extra protection.
The work should take around eight weeks to complete.
Signs are in place to warn people not to walk or climb over the wall, and islanders are asked not to climb on the rock armouring.
Those who will be most impacted by the works have already been notified.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...