(the reply is quite long but bear with me)
Go back and fight the First boss in the castle (the one that spins around) after defeated her continue to walk you should see two path one up one down, take the lower path , hit the statue and the gate will open , walk all the way and you will reach the new level.
After you clear every single level , go collect every treasure item , for certain treasure item you need to go back and fight the boss that you defeated before again , collect until you left only one unfound item , go back and talk to the succubus in the treasure room at the shop behind the red curtain which you can access after defeating the 'final boss' at the top of the tower , she will give you the final item (if she did not give the item to you , go check on the treasure item first then talk to her ）, once every treasure item is all collceted go to the 'memory' option , and talk to the purple hair succubus who is the shop owner ,special dialogue will appear and you will unlocked a portal behind , after enter the portal you will be fighting all the boss , secret boss will appear as the true final boss in the portal
if you manage to reach the secret boss but you die do not worry , go back and click on the portal there will be a third option unlocked for you to click , you will go fight the secret boss straight instead of facing the boss from the beginning again .
