I think you're right. There's more to be done with the concept. I would've given the stage a bit more room to breathe. Here's something I try to do when playtesting my levels that might work for you: I try to beat the whole thing without power ups, to see what works.
With all that said, I like the idea! A thwomp-based castle stage is worth making.
Non-standard stage that's half boss rush, half high-level SMB3 castle. Not really my cup of tea but much too good to not recommend.
In a way you seem like less of a game designer than you do an interior decorator. I mean that as a compliment! There are a lot of interesting things being done visually here. Anyone curious about what this game can look like should try this stage. You have a real eye for detail: the gas lamps in the interior portion would be window dressing in a normal stage, but they're starring players here.
To the stage itself, my big problems are in the beginning: the first jump over the fire bars is too harsh, and I don't like that you've recessed the wrench moles. Other than that, some people might find this too visually busy, but I say if you've got a signature look, embrace it!
I've gone through a bunch of levels and I'll certainly do more. If anyone was looking to give mine a spin, here's the codes again:
The Spring's the Thing: F6DA-0000-0016-3AD4
Big Trouble: 3B43-0000-0016-FFC2
Shell Shock (the TMNT one): 0CCD-0000-001D-9748
Buzzy's Cavern: D2D2-0000-0026-9161
Mole Mountain: 2AB6-0000-002D-182F