This one was certainly perplexing. I knew if I didn’t come up with an answer soon, however, I would lose the contest and…well only God knows what he would have done to me. Well, I would just have to think through it.

In marble walls white as milk…
Lined with skin soft as silk.

White certainly meant something, white on the outside, apparently.

A fountain crystal clear…
A golden apple does appear.

Again, colors seem to be playing a part in this, so if I forget about the actual objects like walls and apples, I would get a white outside, clear inside with a golden something, perhaps?

No doors…
Yet thieves break in to steal the gold!

“An egg,” I said triumphantly.

“Very good, your time was almost up.”

“So it’s my turn then? All right.

‘What force and strength cannot get through,
I with a gentle touch can do.
And many in the street would stand,
Were I not a friend at hand.’”

Almost immediately it said, “I am afraid I do not know; your passage is now free.”