There are some aspects about the
bridge of Khazad-dûm that were left unexplained in LotR IAFAIK.
This bridge is described in the following text:
At the end of the hall the floor vanished and fell to an unknown depth. The outer door could only be reached by a slender bridge of stone, without kerb or rail, that spanned the chasm with one curving spring of fifty feet. It was an ancient defence of the Dwarves against any enemy that might capture the First Hall and the outer passages. They could only pass across it in single file.
This defensive function is rationally explained as long as no alternative entrances to
Moria exist on
the East side (or internal bypasses) since these would invalidate the defensive reason.
This bridge was destroyed completely during
Gandalf's fight with the
Balrog:
Fortresses were rarely built with a sole entrance (pardon if this has already been explains). The postern-gate on
Helm’s Keep is one such example.
These other entrances, though, where usually built to be trivially obstructed, or defended.
Whereas the main entrance/exit to a
fortress of some kind was the only real means to pass an army through the Fortress (either coming, or going). I am more interested in that bridge for other reasons. Because the "Main Entrance" also needed to be large enough to pass something else through, that is
vastly more important than an army:
Food, Water, Tools, Materiel, and Garbage/other Waste.
Now, I suspect that
Khazad-Dûm did not have problems with waste disposal (a combination of deep-pits and furnaces deals with that problem). Likewise, it probably didn’t have many problems with fresh water (Heck! they probably even had hot-cold Running Water. If the Romans did, I expect Dwarfs too, to have it). But food...
When the Ancient Trope saying "An army travels on its stomach." was first spoken, few truer words could be said. And this is
DOUBLY TRUE for
Cities, which is ultimately what Khazad-Dûm was. And by the description of its size, and Gimili’s statements about it, Khazad-Dûm likely had between 250,000 and 500,000 Dwarves in it at its height.
To give an idea of what a
city of 1,000,000+ requires,
daily just to keep up its food supplies, it requires about 5,000 Truckloads/day. Ancient Rome was little different, save that it contained more livestock, poultry, and local gardens to produce seasonal
items, locally. That offset maybe 1/5 of the required "import" of food.
That little bridge isn’t going to be able to handle all of that traffic, coming and going (that is 5,000/day coming, and 5,000/day going).
I suspect that the dwarves had other entrances that were not directly connected to the "Fortress" portion of the city that had such Infrastructure.
Tolkien was marvelous at building the cultures of
Middle-earth, but he often dropped the ball on things like Infrastructure Support for a population, or things like Predator-Prey relations for how big a population would need to be (remember, Humanity and Dwarves are a Prey Population to
Orcs — many more people had to live in Middle-earth than Tolkien immediately reveals due to the necessary Infrastructure needed to support what he does describe,
and to support the populations of feral Orcs that seem to exist).
The bridge cracked. Right at the Balrog’s feet it broke, and the stone upon which it stood crashed into the gulf, while the rest remained, poised, quivering like a tongue of rock thrust out into emptiness.
Even as Aragorn and Boromir came flying back, the rest of the bridge cracked and fell.
What I keep wondering about each time I read this is that this strategic destruction did not prevent
Gollum from following the Fellowship soon after. Not only that; a whole
orc host started the pursuit the same evening. Besides, Aragorn expected them to come, despite the broken bridge:
"The Orcs will not, maybe, come out till after dusk, but we must be far away before nightfall."
So what to make of this...? Does anyone know of any mention of other gates or ways out of Moria to the East, through which this host could have passed?
As I said above, there
MUST BE such ‘gates’ or ‘passages’ into/out-of Moria for the reasons I mentioned above.
In these passages, Tolkien is at least giving us a hint that such passages exist, even if he is not consciously and intentionally aware that they do exist (I suspect he did know they would exist, but the reasons the characters would not have known specifics are legion).
It is the difficulty in getting large numbers of people through such ‘Postern’ Entrances/Exits/Gates that was likely why Aragorn knew their pursuit would be delayed in setting out after them, as the Orcs would need to re-group, and then plan on which such Postern-Gate was most convenient (and in repair/functioning) for their needs. Then they would have to march the distance to it, pass through it in single-file, and then re-group outside of Khazad-Dûm.
MB