"Lock" and "close" in German
I was trying to explain to someone that my door can only be opened with a key, regardless of whether the door is locked or simply closed.
I figured schließen would fail to express that unambiguously though, as it can mean both, and circumvented the issue with
Der Schlüssel wird immer gebraucht, um diese Tür zu öffnen.
Up next in my list is sperren but I only ever hear it used for things that are blocked. DWDS seems to agree:
das Fenster, die Tür sperrt (= schließt schlecht, nicht)
For that case, how could I make the distinction between locking and closing a door made in idiomatic and unambiguous German? That's my main question but you are welcome to cover the subtleties of those popular close/lock verbs here if you fancy. :)