If there's no entrance, then it can't be found.
I see. That makes sense.
Sure people will look for trapdoors, but few will take a pickaxe and start wailing on walls.
Ancient Egyptians invented this technique.
Archaeologists have to pay special attention to seemingly empty graves.
Maybe that's how teleportation ruins were discovered as well.
"Em, if it doesn't work, then we'll return early."
"May the fortune of war be with you.