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This update starts with a HP boost hidden in a smashable...
...and a Meal Ticket (summons a random food item to pick up for later use) and dessert hidden behind the nearby bookcase.
Anyway, now that I have the Gravity Boots, I can do a few more checks in the areas I've already been. Pictured: Me checking a previously out-of-reach room in the Alchemy Lab.
It has a snorkel, which prevents water damage. I don't care if the game calls it a "Holy Symbol," it's a frickin' snorkel, and the fanbase in general seems to agree with me on that.
There's also a ledge I can reach in the top-right corner of Gaibon and Slogra's room, which leads to Power of Mist. Game got my hopes up, but this doesn't actually let me turn into mist.
Since I'm passing through anyway, might as well make a quick side-trip to pick these up. Turns out they're both consumables; the yellow thing just cures poison, though, so who cares?
The nearby puppets also drop a throwing weapon, but I still don't have any reason to care yet.
Back at the big clock, I arrive pretty much just in time to use the top-left exit. I can go back to the Outer Wall whenever I want, so sure!
I get a HP boost on the way to what I can only assume to be the guest rooms; why else would Olrox have his own quarters?
A breakable wall near a poltergeist leads to a long corridor with a few jars at the end, containing two HP boosts and more dessert. Someone in this castle must have a sweet tooth.
After a hallway with two armor enemies which I'm pretty sure are named after their weapons, I unlock the ability to come back here whenever I want! 
Back to the Outer Wall, I get a novelty item from what I can only assume to be the reanimated corpse of Donkey Kong's distant ancestor. Please note: There is only one source of cat damage in the entire game, and I won't be seeing it for a long time.
Granted, I still equip it, because it at least gives INT, but that's the only practical benefit it gives.
It takes a few tries, since it's a pain in the ass to super-jump diagonally in this game, but I eventually manage to get more headgear from a smashable just above the door from the Marble Gallery. It, at least, has a purpose beyond being a joke and boosting INT.
I need to double-check my tracker to realize it, but there's nothing above the elevator switch for me yet. While on my way back to the warp room, that... armor guy with the really big sword drops an Iron Ball. Which Alucard somehow throws. Apparently vampires can pick up wrecking balls and throw them at you?
To the Colosseum, via the poltergeist room in Olrox's Quarters! Because there's apparently nothing else in logic right now...
Investigating the area yields a crappy Shotel. Why would I want this thing?
And yes, I switched back to the Rapier; it's about half range and half DEF.
Plowing through a bunch of skeletons dual-wielding swords (and picking up a Garnet they drop along the way) gets me a consumable. Meh.
Well, that's a good prize for this area: Killing the local boss will give me Form of Mist. Which is, in turn, my access to flight, since I already have Power of Mist to let me stay in mist form. Which will really open up a lot of areas to me, because flight is kind of a big deal.
Smash a hole in the ceiling with a spell to get another mediocre sword on the way.
Well now, that's an interesting drop from the same skeletons that dropped that Garnet earlier. Think I'll give it a try; hopefully it isn't resisted by too many enemies.
HP boost at a door that's supposed to be a shortcut to another area, but is actually my first access to that area because randomizer.
Another hallway full of enemies, another useless consumable. At least the enemies had the decency to drop their dessert along the way...
I pick up some... slightly less terrible armor immediately before the boss room. As in, it's worse than what I'm wearing but at least manages double-digit DEF.
A quick cutscene skip later, I'm fighting a Werewolf and Minotaur. They put up more of a fight than I expected going into this, but I can fly now so it's worth it.
Dropped from that poltergeist on the way to the warp rooms so I can backtrack with my newfound flight. I keep forgetting I even have subweapons, to be honest... Well, at least it gives an across-the-board stat boost to make it worth bothering, right?
First stop in this flight-powered backtracking session is way back at the gates, where I find... the ability to see monster names. Meh.
More importantly, I get that sword I mentioned wanting to save up for, for free! 
Anyway, since I'm in the area, I go through a gap in the ceiling from back when I was heading to the Death cutscene, plow through a Gurkha and a Blade (the latter of which is one of the weapon-named enemies I mentioned from the Olrox warp room hallway- well, technically they both are, but the Blade was actually there; the other guy had a hammer), and find... another throwing weapon. Well, if I find that accessory, at least I can dual-wield them, right?
There's way too much to do right now; I'll have to work on the rest of the checks that just opened up to me next time. I mean, Form + Power of Mist opens up practically the entire top half of the castle!