Ed sticks close to Al, and replies, "Avoid the cheetahs. They're the Three. Got it." The cavern does indeed register as familiar to him, which helps reaffirm that they are on the right path. Upon arriving at the 'door', he mimics Al's knock as best he can.

Cipher Nine frowns and squints at the rock when Ed knocks on it. "Edward? That's a rock."

"Showing up as a door for me. Could be both."