[Lalwendë]

Nice thread!

I think that the difference between the two powers lay in the false pride of the Gondorians and the superior technology and tactics of Sauron's forces. Let's look...

At first men laughed and did not greatly fear such devices. For the main wall of the City was of great height and marvellous thickness, built ere the power and craft of Numenor waned in exile; and its outward face was like to the Tower of Orthanc, hard and dark and smooth, unconquerable by steel or fire, unbreakable except by some convulsion that would rend the very earth on which it stood.

The walls of Minas Tirith are mighty, and believed impregnable. Remember that this is the final sanctuary of Gondor. To get there, any force must first take Minas Ithil and then Osgiliath. The walls are designed to keep forces out at the last. But walls to protect are only as good as the weakness of the force beseiging them. If it's a strong force, then those within can be beaten by other means. And the residents know this:

'Nay,' they said, 'not if the Nameless One himself should come, not even he could enter here while we yet live.' But some answered: 'While we yet live? How long? He has a weapon that has brought low many strong places since the world began. Hunger. The roads are cut. Rohan will not come.'

Sauron's force know this. And they have the technology to force the inevitable despair to come on more quickly. Their seige engines and catapults aren't designed to smash or crush, just to fling lighter, and often firey, missiles at a greater height. They also have more advanced weaponry:

and as the trenches were made each was filled with fire, though how it
was kindled or fed, by art or devilry, none could see.

As soon as the great catapults were set, with many yells and the creaking of rope and winch, they began to throw missiles marvellously high, so that they passed right above the battlement and fell thudding within the first circle of the City; and many of them by some secret art burst into flame as they came toppling down.

The catapults aren't described as small, and presumably with a vast and brutalised army at his disposal, dragging huge war engines across the plain might not be such a problem for Sauron. They are also set up behind those firey trenches so presumably they also do not move once set up.

Therefore, you can assume that Minas Tirith simply did not have catapults which could outmatch them, even given the advantage of height. Why is interesting. Is that because the residents felt safe behind that wall? Perhaps they were unable to place large catapults due to the design of the city (which is very old). Perhaps it was a lack of resources.

The following shows Sauron's tactics again and how they worked. He has ignored the impossibility of breaching the wall and instead used his tech to spread despair and incendiaries over the wall. Some of the garrison have run away sickened at the sight of the heads of their comrades used as missiles, others have had to help put out the raging inferno. There's now hardly anyone to defend the city from those running up to attack it. The only thing that can stop it now is an attack on the rear or flank of Sauron's forces, and nobody expects this.

Fires now raged unchecked in the first circle of the City, and
the garrison upon the outer wall was already in many places cut off from retreat. But the faithful who remained there at their posts were few; most had fled beyond the second gate.
Far behind the battle the River had been swiftly bridged, and all day more force and gear of war had poured across. Now at last in the middle night the assault was loosed. The vanguard passed through the trenches of fire by many devious paths that had been left between them. On they came, reckless of their loss as they approached, still bunched and herded, within the range of bowmen on the wall. But indeed there were too few now left there to do them great damage, though the light of the fires showed up many a mark for archers of such skill as Gondor once had boasted. Then perceiving that the valour of the City was already beaten down, the hidden Captain put forth his strength. Slowly the great siege-towers built in Osgiliath rolled forward through the dark.