cave
宕 centers on a cave: a hollow space under a roof-like overhang. The meaning is primarily a physical cave, with no strong abstract extensions in the given glosses.
宕 combines 宀 (roof) and 石 (stone), suggesting a stone cave or a hollow under a rock overhang. The exact historical development is uncertain, but the component combination fits the cave meaning.
Under a roof-like overhang 宀, there is a stone 石 hollow: that is a cave.
For トウ, imagine a tall stone cave echoing with a deep 'toe' sound when you step inside: toe -> トウ.