Years passed. After the king died above, Gerson mentored the heir and his soon to be wife, since their parents were dead, they drew from his life force instead, but since they weren't biologically tied, they aged much slower than most. Over the course of 200 years, the monsters built a civilization in the cave in which they fell. They did not expand in fear of the cold, nor were they overly confident in their young leader to deliver them to safety, even if Gerson was running the show back then. So they built a door to block out the cold, and the Royal house was build on the entrance to the passageway that lead to that door so that a monster couldn't stumble into the unknown dangers of the outside world.
Back then, the monsters lived in fear, and built traps in order to stop the humans from finding them if they ever decided to follow the monsters down into the cave and finish them off. The greatest minds were charged with the task to create obstacles simple enough for monsters, but complex enough to stump any wayward humans, luckily, it did not seem as though any humans followed them down during that time.
However after 200 years had passed, the monster population slowly restored to something resembling its old population before the massacre that the war brought on. They were outgrowing their home right as Asgore and Toriel were reaching the day they were to be wed.
Ultimately the decision was made that the new King would postpone the marriage until the monsters found their new home. Most of the monsters prepared to leave with the King, but some of the more skittish monsters did not want to leave, and stayed behind.