Ok, so I think I got two endings. One the 'normal' way and the other after discovering all the secrets. Does this sound about right or is there more to come? SPOILERS below explaining the endings I found.
- After clearing the 'boss' room in the well (marked on the map) and leaving the room to the right (where you shouldn't have an exit according to the map.
- The one shown in the video - after finding all the secrets all the special items in the inventory upgrade and you get access to the one gate outside.