When the top level was built, no more could be placed. It was and is the maximal apex.
Madness string 1919
I'm in too much pain for that poor plant to jerk on this one. And then I get looped into thinking about who the hell thinks that's a thriving plant? Ouch. And then my brain starts to ache. I've read fantasy novels with torture pits that weren't as deep or dark as that poor pot.