a 19th century round martello tower fort in londonderry

19th architecture background blue british building buildings century coastal county defenses double dune elevated exterior fort fortifications fortress forts french garrison grass grasses green historic history invasion ireland landmark limavady londonderry long looking magilligan martello military napoleonic nature new northern palmerston point post rainbows remote reserve round sand security sky small staircasei steel stone structure thick tourism tower travel unusual wall walls war wars