Ode to Castlvania question

[Koro]

So I've unlocked all the doors but one, and I can't work out how to unlock it. It's the one on the top left of the castle, by the silver ring clock face boss thing. Anyone know how to open it? I've tried beating all the bosses in the area, I've beaten the clock face boss. Nothing seems to work. What am I missing?

[Ernivus Garveno]

BIG SPOILERS AHEAD That door won't open until you face the final boss of the Ode to Castlevania map. Only way to do so is to use Richter and go all the way up where Dracula should be. A familiar scene will play out and you'll face someone who is not Vlad. Is way easier than it seems, just survive and avoid projectiles. Once you're done with this mostly theatrical boss, end credits show up and once you play the map again it will have expanded once more, with two more sigils to collect, this time on the southern part of the map with some new bosses there. Not only that more than twenty characters appear to unlock! And with the sigils you can go to the door you mention, to face the worst boss in SotN and unlock A Miserable Little Pile of Secrets. Still figuring them out myself, and let me tell you, I hate riddles.

[Arc]

How do I open the door below spawn, in the forest, off map?