cavern
窟 centers on a hollow space inside rock or earth: a cavern or cave.
窟 combines 穴 (hole, cave) with 屈, which likely contributes the sound and the idea of bending or crouching, suggesting a space one must crouch to enter.
The top 穴 is a cave opening, and the bottom 屈 looks like a person bending to enter. Picture crouching down to go into a cavern.
For クツ, imagine putting on your boots (クツ) before exploring a dark cavern: boots -> クツ, and you need them to walk through the cave.
cave; cavern; grotto
den; haunt; hangout; nest; lair
cavern; grotto; rock cave
den of vice; rogue's den
cave; cavern
enchanted cave
treasure mine
The Count of Monte Cristo (novel by Alexandre Dumas)
buried earthen jar that makes sound when water drips into it (garden feature)
She: a History of Adventure (1887 novel by H. Rider Haggard)
cave painting
cave diving
cave bear (extinct, Ursus spelaeus)
very shrewd in preparing a means of escape; being very good at escaping danger
brothel; house of ill repute; red-light district
cave temple
slum
opium den