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"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.

Later I looked it up, finding a multitude of possibilities, many of which, however, seem to have the same double meaning issue, e.g. [_zu-_](https://www.dict.cc/?s=zuschliessen) and [_abschließen_](https://www.dict.cc/?s=abschliessen).

Up next in my list is _sperren_ but I only ever hear it used for things that are blocked. [DWDS seems to agree](https://www.dwds.de/wb/sperren):

> Beispiel:  
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. :)