My party discovered the secret door to the lift their very first trip down to Jzadirune - and I almost had a TPK. The space is rather constricted, they missed the trap, and the hobgoblins heard them. So, the party was trying to rescue a party member from the slime pit (1d6 damage per round), while fighting the hobgoblins with cover all in a rather restricted area.
Let's just say that their first trip to Jzadirune did not last very long - one fight, and then back to the surface to recover with three (out of five) unconscious PCs.
Also, don't be in too much of a rush to have the party go down to the Malachite fortress - there are a number of tough fights - especially if you check to see if the hobgoblins hear combat in other sections of the complex.
My PCs managed to go straight to the Slave Bazaar and started fighting Kazmojen, Prickles, Pyllrak Shyraat and the Hobgoblins. This raised the alarm, and pretty much every other hobgoblin in the Malachite Fortress came to investigate and help to defend their leader. If they had not been 6 PCs along with Fario and Fellian, there would have been numerous deaths.