In order to access the mountaintop, you must climb the 45 floors of the Upper Garmon Mine. The Harvest King lives on top of the mountain. You must speak to the Harvest King once you've reached the "Hero" title and rung all five bells in order to finish the main story. Once you have unlocked the Harvest King, he will create a shortcut that makes it easy to access the top of the mountain. This makes it easy to come and speak to the Harvest King at any point.
There is also a hot spring to regain stamina and this is where you get the blue feather after you get a love interest to 9 hearts. You must also bring your child here if you want to send them over the rainbow for New Game +.