The Spy Quarters is a tower that serves as the Spy's safehouse and clandestine hideout. It can only be built after the kingdom has become renown enough to need a reception hall to publicly handle diplomacy (and, thus, need a spy to privately handle diplomacy...).
Being built so out of the way, few Sims are likely to wander in to the Spy Quarters. Those that do would likely assume the place is simply a guest room or the quarters of a knight, as it appears to be a nobly-furnished but unassuming living area; it has a large four-poster bed, a fireplace, a chamberpot, a large desk, and a small rack for armaments. However, some might notice that the bed isn't placed as close to the wall as it could be and that the bed's curtain is concealing a metal door leading to a staircase...
A large, unused storage room on the second floor serves as the spy's actual work area. The room is furnished spartanly and with function in mind instead of comfort. It contains a writing desk, a training dummy, a table to craft potions and poisons on, and a wine cask. One side of the room has several wall-mounted shackles and cages, one of which still contains a skeleton.
Unusually, the "public" first floor doesn't seem to have any windows while the "secret" second floor appears to have at least five (at least, on the outside).
The Royal Advisor of the monarch is the only person who will enter the secret area of the spy.