Any Building or Edifice can serve as such if assaulted from without, but a true LR was almost certainly constructed long before (perhaps aeons before) the conflict at hand, and may be a Polder of toughened reality, so that the adversary's forces wash against it – at least initially – in vain. The LR may well house the last remnants of the old world, and be a repository of last Magic in particular. The period in which the story is set may well be towards the End of the World, in which case an atmosphere of Belatedness is likely to be overpowering. If the tale is set in more normal times, the LR's defenders are likely to be faithful to the true cause and/or to include a Hidden Monarch. The reader may well expect – until disabused – any LR to incorporate features characteristic of an Edifice, and will almost certainly hope that it contains in particular a Portal at its heart, through which the protagonist is destined to pass in order to redeem himself, or the Secondary World, or both.
LRs are numerous. They include the Great Redoubt in William Hope Hodgson's The Night Land (1912), Aslan's How in C S Lewis's Prince Caspian (1951), the Citadel in Earth's Last Citadel (1943 Argosy; 1964) by Henry Kuttner and C L Moore, Hagedorn in Jack Vance's The Last Castle (1966 chap dos), the City of the Pyramid in Michael Moorcock's The Queen of the Swords (1971), the Redoubt of All Humanity in Graham Diamond's Haven sequence and the Keep of Dare in Barbara Hambly's Darwath sequence. [JC]