close, shut, watch-tower, rest
闋 centers on the idea of closure or ending: shutting a gate, finishing an activity, or coming to rest. The watch-tower sense is a specific structure associated with a gate or closure point.
闋 combines the gate enclosure 門 with 癸, which likely contributes a phonetic element. The character is associated with closing or ending, possibly from the image of a gate being shut.
The gate 門 closes around the inner shape 癸, like a heavy door shutting off a watch-tower at the end of the day. When the gate is closed, everything inside comes to rest.
For ケツ, imagine a gate that closes with a sharp kets sound, like a latch snapping shut. The kets sound cues ケツ as the gate 門 shuts tight.