hi there and thanks so much for taking the time to share your experiences and thoughts about the characters, story and game. Truly this is why I am making these games ^_^. Im going to jump straight into spoilers so I dont forget to address your questions! Be warned spoilers ahead!
The garden, tunnels and fortess are loose references to persona, ego and shadow. Ilyons outer mask of a calm and rational person, his understanding of himself as a person who has always desired to escape his sense of helplessness and his shame of being born somewhat a freak of nature. The door is representative of his minds eye opening to beyond his sense of self. Mikail in his current reality cannot follow him through the door because he is not seeking enlightenment, but was coerced into the ritual. Yet somehow their bond transcends the singular reality and another Mikail who was searching for enlightenment and choose to be there was able to help Ilyon beyond the door. He didnt become a different person, so he has somewhere in himself that same desire to find a higher meaning to his life and existence in the version that didnt pass the door, he just never pursued it. It becomes a curious question, at what point does decisions and experiences completely change who you are? In the case of power, empathy or logic, ilyon is still a variation of himself. But when his parents and cousin live, there is a different person "Caylis" whom Mikail does not recognize. Yet somehow both Caylis and Ilyon ended up in the ritual so maybe they are not so different lol. I'll let you decide ^_~