From placing candles, to throwing win.... weak holy water and to placing seal breakers.
Anyways, after some confusion finally managed to beat this game.
I might need to replay the windmill again cause I haven't solved that one yet.
[couple hours later]
Ah so that's how you solve windmill puzzle. It was simpler than I thought before.
As for secret room, I'm not sure if I've found it but I found unrelated area with unknown symbol.