On a more serious note, if Moria was a trade center for the dwarves why is the path to the west so narrow and hard to negotiate. I can understand a few tight spots like the bridge that Gandalf falls from, but it seems the whole place is difficult to get through. Also, I always pictured the gates on the west as being bigger, but maybe that was just me.
Moria (or Khazad-Dum as it was then called) was not exactly a trade center: it was the *home* of the Dwarves: the oldest of their great cities. They didn't have a lot to do with outsiders, especially during the First Age. In the Second Age, they became very friendly with the Smiths of Eregion in what became Hollin; but after Sauron destroyed Eregion and scattered or slew the Noldor there, the Dwarves shut the West Gate: Sauron laid siege to it for quite some time, but wasn't able to get in.
As another poster pointed out, the Gate Stream used to flow freely through a wooded valley before passing down into Hollin via a series of falls; along this was a road which conected Eregion and Khazad-Dum. By the time of the War of the Ring, the whole region was desolate, the Gate Stream had been dammed up and turned into a lake with the Watcher inside it, and the road had pretty much ceased to exist.
Tuor