IPA: /ˈwɔːrdər/
KK: /ˈwɔrdər/
A person who guards or watches over a place, especially a prison or a gate.
The warder checked the identification of everyone entering the prison.
Warder → It is formed from "ward" (from Old English *weard*, meaning to guard or protect) and "-er" (meaning a person who). A warder is a person who guards or protects something.
Think of someone who is a 'guardian' ('ward') — the '-er' indicates that this person is actively guarding or protecting.