KanjiJLPT N3
Detain

detain, fasten, halt, stop

Mnemonic

A tent is pitched at the edge of a field to detain captured soldiers. A guard drives his sword straight through the tent flap into the soil, pinning it shut. No prisoner can cross the field—the sword anchors the tent like a lock, holding everyone firmly in place.
Additional thoughts
Picture the sword stabbing down through canvas into dirt—the tent becomes a cell, the field an impassable yard. The sword is both weapon and fastener, embodying the 'pin down / hold in place' sense of Detain.
Quick recall
A sword pins a tent shut at the field's edge to detain prisoners inside.

Details

The keyword for 留 is detain. This kanji conveys the idea of holding something in place, keeping it from moving on, or causing it to stay. It encompasses meanings such as stopping, fastening, or remaining in a particular location. The sense extends to situations where someone or something is kept back, whether physically restraining a person, pinning something down, or staying behind while others move forward.
On'yomi
りゅう、る
Kun'yomi
と.める、と.まる、とど.める、とど.まる、るうぶる