The secret torture chamber can be accessed through the castle basement, which is where the count keeps his darker secrets (particularly concerning argonians). First locate an open barrel near the back wall of the basement - this is actually quite clever because guards rarely check ordinary storage containers. Inside that barrel, you'll find a lever that reveals a hidden passage when pulled.

Once you're in the secret corridor, look for a second lever that opens up more of the passage (this design was common in noble estates, allowing for quick escapes during sieges). You can either use the secret room key or, if you're confident in your lockpicking skills, pick the door lock to enter the torture chamber itself. From there, you'll have access to the countess' private quarters, which is essential for the thieves guild quest.

A word of caution about timing - the countess retires to bed around 11 pm, making that your ideal window for sneaking in (guards are actually a bit more predictable during night shifts). The jewelry box you're looking for sits right beside her bed, but remember that even sleeping npcs can wake up if you make too much noise. Maybe interesting: this secret passage system actually connects to several other quest storylines, showing how the developers layered multiple narrative paths through the same spaces.