Albert Barnes Commentary Judges 3:25

"And they tarried till they were ashamed; and, behold, he opened not the doors of the upper room: therefore they took the key, and opened [them], and, behold, their lord was fallen down dead on the earth." — Judges 3:25 (ASV)

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).