1 Dwalin: To get to the door to the sikkret dwarven kingdom we have tae climb oop high intae the moontains. {translation: To get to the door to the secret dwarven kingdom we have to climb high up into the mountains.}
2 Mordekai: Why on Earth would the entrance to an underground kingdom be at a high altitude?
3 Dwalin: The Dwarf King wanted a sikkret undergroond kingdom wuth a view. {translation: The Dwarf King wanted a secret underground kingdom with a view.}

I had some other possible reasons why they'd want to the door to the underground kingdom high up in the mountains.

"We originally had a door low down, but it took six months to pump the kingdom dry the next time the Mighty Elusian River flooded."

"You want to go in at the bottom? That's like entering a human town through the sewers. Actually, not just like it..."

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2015-11-23 Rerun commentary: I imagine the underground kingdom with a view is something like the Eiger Windows.

The Eiger is a huge mountain in the Swiss Alps, with a height of almost 4000 metres. One of the most amazing things about the Eiger is that it has a railway tunnel bored inside it, to allow people to travel to Jungfraujoch. And there are two stations inside the Eiger, where people can get off and go to one of the Eiger windows to admire the view. These are panoramic windows set right into the side of the mountain.

Check out some of the photos of the windows, both from the inside and the outside, that you can find using Google image search. These things are truly mind-boggling. You can be safe and comfy inside, peering out a window in the side of an actual Alp, while mountain climbers are struggling up the rock right outside.