"The story was that he built his tunnels with slave labor and, in the interests of secrecy, killed the workers when they were finished."
— The New Yorker, "Crime, Drugs, and Politics in Guadalajara, Mexico"
Helpful Tip: Keep your secret lair a SECRET.
A Secret Lair isn't so secret with a couple of hundred ex-construction workers wandering the globe, blabbing to bar patrons about the secret shark trap you built into your underwater grotto. That's why it's vital to mind Helpful Tip #12:
ALWAYS KILL OFF YOUR CONSTRUCTION CREW ONCE YOUR LAIR IS COMPLETE!
Never work for an alchemist, lad. As soon as the job's done, you'll surely be found floating facedown in the Moat to keep the secrets of what you've just built. However much gold they offer you, it just isn't worth it. Believe me.
—Saying among builders, Septimus Heap: Darke