The Secret Door
The Secret Door is a hidden entrance on the western side of the Lonely Mountain, constructed by the dwarves of Thror's era and marked on Thror's Map by a rune and a pointing hand. It is a small passage, five feet high and three feet wide, designed to be indistinguishable from the natural rock face, and its discovery and opening become the central objective of Thorin and Company after they reach the Desolation of the Dragon. The door represents the only viable approach to the dragon's lair, as the Front Gate is both exposed and regularly used by Smaug, and it is the key to the entire quest for the dwarves' lost treasure.
Design and Concealment
The Secret Door was built by the dwarves at the height of their wealth and skill, a passage "straight as a ruler, smooth-floored and smooth-sided, going with a gentle never-varying slope direct—to some distant end in the blackness below." Its entrance was made to look exactly like the side of the Mountain, a common dwarven method of concealment. The door was too small for Smaug to enter, even when he was a young dragon, which is why it remained secret and unused for years. The map indicated that the door was "five feet high the door and three may walk abreast," and it was secured by a magic that could only be unlocked under specific celestial conditions.
Discovery and Opening
After days of fruitless searching on the western slopes, Fili, Kili, and Bilbo Baggins discovered a narrow track that led to a little steep-walled bay, grassy-floored, still and quiet. At its inner end, a flat wall rose up, smooth and upright as masons' work, but without a joint or crevice. They had found the door at last. The key to opening it was the moon-letters on Thror's Map, which Elrond had read: "Stand by the grey stone when the thrush knocks, and the setting sun with the last light of Durin's Day will shine upon the key-hole." Durin's Day is the first day of the dwarves' New Year, when the last moon of Autumn and the sun are in the sky together. As the sun set on the last day of autumn, a red ray of sunlight escaped through a rent in the cloud and fell on the smooth rock-face. An old thrush, who had been watching, gave a sudden trill, and a flake of rock split from the wall. Thorin Oakenshield then used the silver key that had been given to him by Gandalf, fitting it into the hole. The key turned with a snap, and a door five feet high and three broad was outlined, swinging inwards without a sound. Darkness flowed out like a vapour from the hole in the mountain-side.
Role in the Quest
The Secret Door was the only means by which the dwarves could enter the Mountain without confronting Smaug directly at the Front Gate. It was through this passage that Bilbo Baggins made his two perilous journeys into the dragon's lair, first to steal a cup and later to converse with Smaug and discover his weak spot. After Smaug's rage, the door was shattered and blocked with broken rock, trapping the dwarves inside. Neither the key nor the magic it had once obeyed would ever open that door again. The Secret Door thus served as both the entry point for the burglary and the final prison from which the dwarves had to find another way out, leading them to the Front Gate and the subsequent events of the Battle of Five Armies.
