Board Thread:General Discussion/@comment-29546017-20160809202024/@comment-27701762-20160809205445

BoltieXRektin wrote: One-way exit means it is the only exit, not just exit. No, "one-way" means that it only operates in one direction. It's like a one-way road. "One-way" never means "only" in the sense that it is the "sole" exit.

Sans is a boss monster too, Papyrus included. While Sans and Papyrus are "bosses," they are not Boss Monsters. Boss Monsters are a specific class of monster whose soul exists for a few seconds after they die. All other monsters disintegrate into dust, soul and body together. Both the story in Waterfall and Gerson remark that Asgore and Toriel are special classes.

There is no entrance to the Ruins excluding the door

There is no entrance that we see. But what we see of the Ruins is not its entirety. Near the end of our journey through the Ruins we can see the view of a large city, with buildings and all. There is a lot of the Underground that we don't see, and it is entirely plausible that there is another entrance/exit to the Ruins.

Asriel is at the flower bed where you fell down, and that is the Ruins, and Asriel is not capable of magic without the SOULs. Though Flowey appears to be able to travel underground, which may mean he doesn't need to use a door.

And of course, if there is another entrance, he could conceivably have used that.