A person stationed to keep watch or guard a place, often in a military or security context.
城門の前に番兵が立っていた。
A sentry stood in front of the castle gate.
番兵は夜通し見張りを続けた。
The guard kept watch all night.
Compound of 番 (ban, 'watch' or 'guard duty') and 兵 (hei, 'soldier'). The exact historical derivation is uncertain, but the combination naturally yields the meaning 'guard soldier'.