Board Thread:General Discussion/@comment-30664268-20170420225552/@comment-26006155-20190503145751

User1204 wrote: No one could've placed it there. The ruins were locked since Asriel's death. And yet, Toriel has opened that door several times, to let humans though.

And that's not all. Remember how Toriel said she was going around looking to see if anyone had "fallen down"? All the monsters delivered to Alphy's lab had Fallen Down. And some of them were Vegetoids, creatures Frisk can only encounter in the Ruins.

While only burrowing creatures and ghost have the power to move between the Underground and the Ruins at will, Toriel can open and close that door as often as she pleases.