I suppose I could just leave it at that, but then all of the non-Tolkien/Roman-History-geeks out there will be scratching their heads, and that would be cruel, so...
Brief review of Middle Earth geography:You have this river, Anduin, running down the middle of everything, rather like the Mississippi. Source is somewhere in that nasty home-of-the-goblins northern mountain range whose name I forget, it flows south between the Misty Mountains and Greenwood/Mirkwood, passes by Khazad-dum and Lothlorien, passes the eastern end of Rohan, curves around the White Mountains and then passes between Gondor and Mordor and thence out to the sea. And right at about that point, there's this big-ass ruin of a city, Osgiliath, that makes absolutely no sense.
It's right in the middle of the river, completely indefensible and insanely huge. What is it doing there? Why is it so big? What the hell were they thinking?
It puzzled me enough on the first reading back when I was 12, but became ever more jarring when I saw Peter Jackson's adaptation, because he has the city being really freaking huge — ruins going on as far as the eye can see in all directions — which, to be fair, is not entirely unfaithful to how it's described in the book.
I mean, it's a cool set. Cities, abandoned or otherwise, are completely horrible places to fight battles --- way too many places to hide and strike from. Suspenseful. Creepy as hell. And if you're going to have a shout-out to Lost Ancient Grandeur you may as well do it in style.
It's not like Gondor at its height would ever have needed a port this large. Never mind that ports only make sense if you have stuff further up the river — and there wasn't in this case since the rest of Arnor was on the other side of the mountains — or if you have a Rest of The World to trade with, never mind that at the time this place was built (end of the 2nd age), the only other place worth sailing to (
Now perhaps the Rule of Cool is sufficient explanation — not everything has to make sense — but it's hard to shake the feeling that something else is going on here.
It helps to recall that Tolkien wanted to create his own mythology for Britain and that he would from time to time insist that Middle Earth was intended not to be a wholly imaginary world but in at least some sense a previous epoch of this one.
Reading Steven Baxter's Emperor, book 1 of his alternate history Time's Tapestry series — which I'll plug even though I really didn't like what he did in the last book and even though it did scream a little bit too much of "Yes, I did The Research!" — I was struck by his depiction of Claudius' invasion of Britain (43 AD) and, in particular, the description of the establishment of Londinium and what happened when the Romans withdrew 400 years later.
In short, the army needed a place to cross the Thames, and here was this magic spot, just far enough in, where the tidal estuary became narrow enough to ford, but not so far upstream that you had to deal with an actual river flowing fast and deep. And, gosh, there really ought to be a settlement here — prior Celts and Basket People wouldn't have seen the point, so there wasn't anything there before. We're also going to need a provincial capital, we have this handy corps of engineers along with us, and being Romans, we're not going to do things by half measures, so … Instant Big-Ass City, that, over the next several decades built up sufficient port capacity to handle trade with the rest of the empire, Britain at the time having significant mineral wealth to export.
It was not the most defensible site, but this didn't matter a whole lot since the Roman conquest never met more than token resistance. And once Hadrian's wall went up to block the main invasion route from the north, and what with the island itself being Rather Hard to Get To by sea, nothing really needed to be that defensible anymore.
This, of course, changed in a big way within a few centuries, once the various and sundry Germanic tribes figured out how to use triangular sails, and the North Sea went from being a formidable natural barrier to being a rather useful highway -- not quite the huge "Kick Me" sign that it effectively became by the time the Vikings got going, but navigable enough for the Angles/Saxons/Jutes to get there and make the countryside sufficiently dangerous to inspire the Romans to abandon their isolated country villas and other indefensible settlements.
Somewhere around 410 AD the empire throws in the towel, the legions go home, yet more random tribesmen who know nothing about building and maintaining large stonework civil engineering projects show up looking for places to live, the former-Romans who do Know Stuff are driven into the hills, Londinium goes from being mostly pointless to being completely pointless, and the city then lies completely abandoned for the next 400 years — a dark age from which all kinds of legends later emerged (cf. Beowulf, King Arthur, etc...) — until Alfred the Great, having finally beat off the Vikings and reunited most of the south end of the island, decides this would make a Really Impressive Capital and is bad-ass enough that nobody is going to even think about attacking it.
Meaning if you're going to do a mythology for Britain, you can't do it without the Ancient Empire, and you really need the Monster Abandoned City to seal the deal. Now go look at the Middle Earth map ... same magic spot in the river. I think we're done here.