I always thought the Superman’s key to the fortress was so huge, a man should just be able to walk right thru the huge keyhole. And, why disguise it as an aircraft beacon, only Superman could lift it anyway.
I remember a comic when Superman found himself far, far in the future when our sun was a red giant and he lost all his powers. All that was left on earth were robots of Lois, Jimmy and the Chief. He had to journey to his Fortress of Solitude (there wasn’t any water left, but there was “dehydrated” water capsules, natch), and he climbed into the lock.
I forget how he was able to get back into his own time.