Dark Elf Watchtowers

Naggaroth is a land in little danger of invasion. Few enemy fleets survive more than a few hours in its corsair-infested waters, let alone long enough to close with the jagged coast and land troops. The only real dangers come from the Lustrian legions to the south and the hordes of Chaos that roam the frozen wastelands to the north.

The Dark Elves consider that they need no fixed defenses against the former, for the Lizardmen stir themselves to meaningful assault but infrequently. The latter, however, have proven so undeterred by their appalling losses and ignominious defeats that the Naggarothi have founded many great citadels to hold the hordes of Northmen at bay. Each of these Watchtowers is the size of a city, provisioned to withstand years of siege and garrisoned with a great host of Naggaroth's foremost warriors. Thus have threats from the Realm of Chaos been held at bay, at least for now...

Source

 * Warhammer Armies: Dark Elves (8th Edition) -- pg. 13