βασιλεία τῶν οὐρανῶν

The Greek phrase βασιλεία τῶν οὐρανῶν consists of βασιλεία (basileia) and οὐρανός (ouranos).

See the explanation of the word βασιλεία (basileia) here.

As described there, the appropriate translation of βασιλεία (basileia) within the context used by Jesus would be "sanctuary."

The latter word - οὐρανός (ouranos) refers to, “the region above the sidereal heavens, the seat of order of things eternal and consummately perfect where God dwells and other heavenly beings” according to Thayer’s lexicon.

Thus, appropriate phrases to capture the Greek would include, "shelter of the spiritual realm" or “sanctuary of the spiritual realm.”