What is the fine-grained topic of the following text: Behind [Mount Doom] there hung a vast shadow, ominous as a thunder-cloud, the veils of Barad-dûr that was reared far away upon a long spur of the Ashen Mountains thrust down from the North.... [All the Dark Power's] great stronghold, gate on gate, and tower on tower, was wrapped in a brooding gloom. Only return the topic.
The Dark Power's stronghold, Barad-dûr, in the Ashen Mountains