The cats knock on a door with a Wizard of Oz-style guard window that can be opened seperately from the door itself.

Sound FX: knock knock

The guard window opens to reveal a giant mouse. He is huge and gruff and toughlike and could easily take the cats in a fight.

Sound FX: swing

Doormouse: Yeah?

The cats blink.

Ferdinand: Are you a...mouse?

Doormouse: I'm the doormouse. What's it to ya?

Oolong: Hi! I'm Oolong.