Albert Barnes Commentary Exodus 21:33-34

"And if a man shall open a pit, or if a man shall dig a pit and not cover it, and an ox or an ass fall therein, the owner of the pit shall make it good; he shall give money unto the owner thereof, and the dead [beast] shall be his." — Exodus 21:33-34 (ASV)

The usual mode of protecting a well in the East was probably then, as now, by building around it a low circular wall.