When Durin saw the door, his first impression was as though he had traveled back in time to the past when many cities in the Middle East had doors just like this, with two or even three layers.
It's said to be anti-theft, but everyone knows it's really to keep out small-time burglars. For someone like Durin, who's genuinely out to wipe out your entire family, even five doors wouldn't make a difference. Even if your little building has no windows and the whole structure is designed to resist breaking and entering, Durin could still find a truckload of Fish Blasting Tools and blow the entire building and the unfortunate souls nearby to kingdom come.
As for whether there would be innocents involved, given Durin's targets, most of those around him were scum and vermin. The issue was always about the radius of destruction not being large enough.
