lack, gap, fail, imperial palace
闕 centers on an absence or opening: a gap, a lack, or a failure where something should be. The imperial palace sense is a specific historical application, referring to a gate or watchtower that marks a gap in the palace wall.
闕 combines 門 (gate) with 欮, which likely contributes the sound. The character originally referred to a gap in a gate or wall, later extending to abstract lacks and failures. The exact historical development of 欮 is uncertain.
The gate 門 has a gap 欮 inside, like a missing panel. This gap represents a lack or failure, and historically the watchtower gate of the imperial palace.
For ケツ, imagine a gate with a gap so wide you can kick a ball through it: kick -> ケツ, and the gap lets the ball pass.
Imperial palace
imperial palace gate; imperial palace
palace gate; imperial palace
palace gate; imperial palace
before the Emperor; in front of the Emperor
empty government position
gap; hole
unstitched, open side of some traditional Japanese clothing; clothing with such an open side
robe worn by military officials with a round collar, unstitched open sides and no ran