Two of Orkney's famous "Churchill Barriers" have been listed by Historic Environment Scotland.
The concrete causeways were built during World War Two to stop enemy ships and submarines entering Scapa Flow, where the bulk of Britain's naval fleet was based.
Winston Churchill ordered their construction after a German U-boat sank the battleship HMS Royal Oak in 1939.
Today they provide a road link from the Orkney mainland to South Ronaldsay.
There are four barriers in total, two of which will be listed at Category A, the highest status, meaning they are recognised as being of national or international importance.
Elizabeth McCrone, head of designations at Historic Environment Scotland, said: "Travelling through Orkney now, it's hard to imagine the islands as they were in late 1939, as a bustling naval base, living in constant fear of German U-boats getting past the inadequate block ship and net defences.
"It must have come as some relief when Winston Churchill gave the order to construct the barriers.
"Nowadays, the original function as defensive barriers is a distant memory, but their importance remains.
"They serve as a tangible reminder of Orkney's past. The listing recognises their fascinating history and will help to keep the features which make them a unique and significant part of the island's history, while still ensuring that they continue to be a vital transport link between the islands."
Barriers 3 and 4 are listed, but no listing will be given to Barriers 1 and 2 at present because of longstanding development proposals involving tidal energy.
James Stockan, chairman of Orkney Islands Council's development and infrastructure committee, said: "The Churchill Barriers are as important today as when they were first built, providing lifeline links between three of our inhabited islands.
"They are recognised worldwide as unique monuments that serve as a powerful reminder of Orkney's wartime past.
"It is fitting that those listed by HES now have the same status as other historic structures such as St Magnus Cathedral, Balfour Castle and the North Ronaldsay sheep dyke.
"Barriers 1 and 2 are, of course, equally important. It is welcome that we have time to explore interest among developers in the two Barriers forming part of a tidal energy generation project, along with ways to address wave overtopping during severe winter weather."
The causeways prevented access from the east into the four channels leading to Scapa Flow.
The were designed to withstand a 4-5 knot tidal current, with wire cages or baskets filled with broken rock dropped into the water.
A road carriageway was then made with aggregate dumped over the causeway base and concrete blocks laid on top.
In total, all four barriers required about 250,000 tons of stone rubble and 66,000 concrete blocks.
When the Royal Oak was torpedoed in the early hours of 14 October most of the 1,200 crew were asleep below deck
Under the command of Günther Prien, the submarine U47 had slipped undetected into the sheltered water of Scapa Flow.
A first salvo of three torpedoes was fired from the U-boat.
Two missed but the one that struck caused those on board Royal Oak such surprise that many assumed the impact was an internal problem and not an attack.
A second salvo failed to find its target, before a third saw all three strike the battleship.
Less than 15 minutes later, the battleship disappeared beneath the water, claiming the lives of 834 seamen, more than 100 of them "boy sailors" aged under 18.
Prien returned home to a hero's welcome - but perished with his crew when U47 disappeared in the North Atlantic in 1941.
Earlier this year a faulty torpedo from the submarine was found on the seabed in Scapa Flow and made safe by Navy divers.