How did the killer enter?

Hint 1 of 16:

Well, there was no forced entry on the door.

Hint 2 of 16:

And the stairs are incredibly creaky, so no one could have crept up them
without someone knowing.

Hint 3 of 16:

So maybe they didn't get into Robner's room via conventional methods.

Hint 4 of 16:

Really, the other possible entryway is pretty obvious.

Hint 5 of 16:

Check out the rug in Robner's room.

Hint 6 of 16:

Mud, huh?  Maybe the killer had their shoes in some mud.

Hint 7 of 16:

Now look at the balcony.

Hint 8 of 16:

Look at the railings.