In a word or three: I love this. It's fun, as simple or complex as you need it to be, and the twist at the end is nasty. However, I must agree with valadaar that relying on the PCs to be master architects is a pitfall. My current group is a wizard, a druid and a rogue (NPC) -- no real fodder for the plot hook to work.
It's possible for the party to answer an advertisement for the job, and then bluff their way into winning the contract. It's also possible that it could be a case of mistaken identity (the master builders weren't in court at the appointed hour, and the nervous butler ushers the PCs into the audience without asking too many questions). Or what about another contest, of sorts -- to choose the architects for the maze?
In short, I love the premise but would really like to see some more plot hooks to help bring this into an ongoing game.