A key - literally, “an opener.” Probably a wooden instrument with which they either lifted up the latch within, or drew back the wooden bar or bolt. The chief officer of Eglon’s household probably had a second key (compare Isaiah 22:15, Isaiah 22:20-22; Isaiah 37:2).
Related Commentaries of Judges 3:25
Judges 3:25
25 And they tarried till they were ashamed: and, behold, he opened not the doors of the parlour; therefore they took a key, and opened them: and, behold, their lord was fallen down dead on the earth.