Sorry, I think it’s four levers. Not switches.
If I get the time later, I’ll try to have a run through it myself because my memory is a bit hazy on this. The first level is rather more complex than the others, because I wasn’t sure at first exactly what I was going for.
This isn’t really giving anything away, but I’ll spoiler it anyway...
Each level contains a key of essence, a skeleton key, and gem-based key in all except two levels. The skeleton keys are optional, but useful for backtracking. All the others are essential.
What is not obvious (unless you’re mapping as you go) is that levels 3/4 and 8/9 are side by side and not up/down, and are separated by a normal portcullis opened with a brass key (which is why there are two levels without gem-based keys).
The reason I’m mentioning this is that the area you’re in contains an essence key (which you’ll need later), but you’ll need the emerald key to access the stairs.