Birdwatching from the bunkers

Alderney, Channel Islands: From the wartime fortifications, sentries could look down over a cavalry of white horses and sinuous, sinister currents

As birdwatching hides go, the sea-facing wartime bunkers at the north-west end of Alderney are rotting, indestructible curiosities. They are concrete barnacles bolted on to a solid Victorian fort of granite blocks, which was originally built to guard against a French invasion. A hundred years later, the German army of occupation used slave labour to construct the super-fortified extension they called Sti1/4tzpunkt Ti1/4rkenburg - Strongpoint Turk's Castle.

Steadily and incrementally, the elements have attacked these armoured cells in the 70 years since abandonment. In each dank chamber, salt-laden sea spray has bleached wooden shuttering and shrunk the timbers into pieces of fixed driftwood riddled with woodworm. Iron lintels, brackets and other attachments are rust-brown, corroding into flaky layers as if they were puff pastry. But the ugly, metre-thick walls have proved durable and, like a Norman castle, might last for another thousand years.