I´ve played through the game twice. On the second run i found all the bosses and got the secret final boss and, i assume, the "real ending". After that, you can return to the map and find a marker before the first stage. Open it, and you´ll see only a door. I´m curious so i tried everything. I tried looking through the files and held some buttons for longer because i assumed it would be a reference to the filename "Holdthedoor". But it didnt budge. Sadly theres absolutely nothing on the web about this ...

One the better indie games I've gone through in thousands of indie games.