As I base my Walkthroughs and videos on my own games and experience, it was left to Robert to tell me that one of my statements was 'inaccurate', that a Circle Portal is created simply after you confront any Boss. That actually makes perfect sense, but since I never allowed myself to be defeated when I confronted any Boss, I simply wrote that the Circle Portal would be opened after your character defeated the Boss Monster.
I will have to start a new game to confirm this myself because I have defeated all the Bosses at least once in my current RFF game, but as Robert's comment actually is logical, I am going to accept it as true and revise my guides accordingly.
It is useful for players to know that the first confrontation, whether successful or unsuccessful, will create the Circle Portal. As my own character WAS successful, I never stopped to discover if the mere confrontation and not the defeat would be sufficient.