Oct 26 2019, 12:19pm
As to why both the groups go specifically towards Black Gate, it's simple because there is good road approach, old numenorean roads connect before Moannon, one eastern road (a portion of it build north of Ered Lithui leading east, never finished but which nonetheless helps eastern invading armies in earlier centuries, mentioned in UT in regards to wars with Wainriders near thousand years earlier: "The road through Ithilien was still in full repair as far as the Morannon; and there it met a road going north towards the Dagorlad, and another going east along the line of Ered Lithui. [Neither of these roads is marked on the maps to the Lord of the Rings.] The eastward road extended to a point north of the site of Barad-dûr; it had never been completed further, and what had been made was now long neglected. Nonetheless its first fifty miles, which had once been fully constructed, greatly speeded the Wainriders' approach.") and southern one coming through Ithilien lead to Morannon, the Black Gate is also major entrance to the Black Land, one of the few places where huge armies can cross and move easily from and to Mordor. Minas Morgul may be less useful for mortal men because it's mentioned that:
"..because of the evil that dwelt in the valley, where the minds of living men would turn to madness and horror...."
Morgul Vale is very inhospitable for mortals, it would certainly affect Sauron's human allies and servants, probably except some few who would not give in to terror. Or maybe simply because that's the order they received, Sauron was marshaling armies in two places in Udun enclosed by Morannon and Minas Morgul under command of Nazgul, plus the last reinforcements coming from the south and the fleets send by sea. Gollum talks a bit about it, and since our dear Smeagol has surprisingly deep knowledge we may believe him I think:
"`O no, not empty! ' whispered Gollum. `It seems empty, but it isn't, O no! Very dreadful things live there. Orcs. yes always Orcs; but worse things, worse things live there too. The road climbs right under the shadow of the walls and passes the gate. Nothing moves on the road that they don't know about. The things inside know: the Silent Watchers.'
`So that's your advice is it,' said Sam, 'that we should go another long march south, to find ourselves in the same fix or a worse one, when we get there, if we ever do? '
`No, no indeed,' said Gollum. `Hobbits must see, must try to understand. He does not expect attack that way. His Eye is all round, but it attends more to some places than to others. He can't see everything all at once, not yet. You see, He has conquered all the country west of the Shadowy Mountains down to the River, and He holds the bridges now. He thinks no one can come to the Moontower without fighting big battle at the bridges, or getting lots of boats which they cannot hide and He will know about.'
'You seem to know a lot about what He's doing and thinking,' said Sam. `Have you been talking to Him lately? Or just hobnobbing with Orcs? '
'Not nice hobbit, not sensible,' said Gollum, giving Sam an angry glance and turning to Frodo. 'Sméagol has talked to Orcs, yes of course, before he met master, and to many peoples: he has walked very far. And what he says now many peoples are saying. It's here in the North that the big danger is for Him, and for us. He will come out of the Black Gate one day, one day soon. That is the only way big armies can come. But away down west He is not afraid, and there are the Silent Watchers.'"
Morgul Host was composed mostly of Orcs as they are less likely to break living in that 'dead city', more used to it's 'supernatural aura of horror'. Besides Minas Morgul might have been already full :).
And out of the gate an army came.
All that host was clad in sable, dark as the night. Against the wan walls and the luminous pavement of the road Frodo could see them, small black figures in rank upon rank, marching swiftly and silently, passing outwards in an endless stream. Before them went a great cavalry of horsemen moving like ordered shadows, and at their head was one greater than all the rest: a Rider, all black, save that on his hooded head he had a helm like a crown that flickered with a perilous light. Now he was drawing near the bridge below, and Frodo's staring eyes followed him, unable to wink or to withdraw. Surely there was the Lord of the Nine Riders returned to earth to lead his ghastly host to battle? Here, yes here indeed was the haggard king whose cold hand had smitten down the Ring-bearer with his deadly knife. The old wound throbbed with pain and a great chill spread towards Frodo's heart.
At that moment the Wraith-king turned and spurred his horse and rode across the bridge, and all his dark host followed him. Maybe the elven-hoods defied his unseen eyes, and the mind of his small enemy; being strengthened, had turned aside his thought. But he was in haste. Already the hour had struck, and at his great Master's bidding he must march with war into the West.
Soon he had passed, like a shadow into shadow, down the winding road, and behind him still the black ranks crossed the bridge. So great an army had never issued from that vale since the days of Isildur's might; no host so fell and strong in arms had yet assailed the fords of Anduin; and yet it was but one and not the greatest of the hosts that Mordor now sent forth.
Frodo stirred. And suddenly his heart went out to Faramir. 'The storm has burst at last,' he thought. `This great array of spears and swords is going to Osgiliath. Will Faramir get across in time? He guessed it, but did he know the hour? And who can now hold the fords when the King of the Nine Riders comes? And other armies will come. I am too late. All is lost. I tarried on the way. All is lost. Even if my errand is performed, no one will ever know. There will be no one I can tell. It will be in vain.' Overcome with weakness he wept. And still the host of Morgul crossed the bridge.
Then at a great distance..., he heard Sam's voice speaking. 'Wake up, Mr. Frodo!'....
There was a dull clang. The gates of Minas Morgul had closed. The last rank of spears had vanished down the road....
'Wake up, Mr. Frodo! They're gone, and we'd better go too.'
It took some time before the whole army marched out, Frodo managed to take a nap in the meantime :). Anyway that's as far as the source material says about military movements of various troops.