Of course there should always be secret escape routes from the villain's lair. No self-respecting villain would build a lair without a way of getting out when things go horribly wrong (as they inevitably will - it's all part of being a villain).

Additionally there will be various ducts and chutes and tunnels and things leading all over the place. In fact, some villains have been known to use the ducts as the primary means of getting around, and merely utilise the main corridors for air conditioning. That'll fool anyone trying to sneak in via the ducting!

Transcript

Leia: Looks like you managed to cut off our only escape route.
Han: I have dozens more! Han Solo, daring space pilot at your service. Your Highness.
Chewbacca: Her name is Princess. She's not a princess. Although she could easily be mistaken for one, of course.
Luke: {into radio} 3PO, are you there?
C-3PO: Yes. Guarding the ship. No one's taken it.
R2-D2: Yet.
Luke: Are there any other ways out of here?
C-3PO: There's a chute you can go through. On your left.
Han: Aha! A good old laundry chute! That was route number five.
Luke: Is it safe?
[SFX]: < doot bip-jing >
C-3PO: Artoo says it's perfectly safe—
[SFX]: Pow! {Leia shoots the chute hatch open}
Leia: Into the laundry chute, flyboy!
[SFX]: Jump {Leia dives in}
[SFX]: < bedoop >
C-3PO: What's that, Artoo? Oh, for droids?