From Oblivion, we all know that Mehrunes Dagon's invading armies consisted of Dremora Battle Clans, Scamps, Tamed Clannfear, Xivilai, Atronachs, Daedroth (supposed to belong to Molag Bal), Spider Daedra (supposed to belong to Boethiah), and Massive Dwemer-like Warmachines.

However, his army is missing some very important elements-

Cavalry. A key player. Something Dagon lacked, but the Empire (canon) has. Possibly the most important part of the army.
Navy. We never saw any of the seas, but he must have had a Navy. If he was truly conquering Mundus, his forces would have been everywhere, not just Tamriel. Akavir would have been invaded as well. (unless he was to capture Tamriel first, lest the white gold tower stop his advance)

I noticed the same thing with Jyggalag. He's lacking a cavalry.

I don't think we were shown and accurate representation of Dagon's armies.

And what do you think Dremora cavalry would be like?

He doesn't need it.

OB gates = you only need to fight sieges. Cavalry is rubbish in a siege. Cyrodiil is mostly landlocked, explaining the lack of boats.

Why would he have Calvary and Navy when none of those are important in Oblivion? The Daedra don't adhere to our brand of military sense, and if you notice, Cyrodiil (both in game and lore) is not fit for Calvery anyway.

There is nothing in game about Dagon invading all of Mundus or even Nirn. Only Tamriel was mentioned.

And he didn't need a navy.

The Roman Army always gave cavalry the short shrift. Didn't really hurt them until they went up against all-cavalry armies like those of the Parthians. In the period before saddles and stirrups, cavalry were really only good for harassing the enemy, chasing down a routed enemy, scouting, and whatnot. In the heat of battle they weren't of much use.

The real question should be: Why did he need siege engines? He could have just opened portals right inside of cities, like he did in the Imperial City.

That is the real question.

Mounted troops would still help a lot. You'll recall most of his gates were in the counrtyside. I can't imagine what a Dremora would ride though.

I'm not really convinced Calvary troops would have really made a difference, and as far as we know Dremora don't even have calvary..

Archers on walls are even more effective when you're surrounded by those walls.

I'm under the impression Gates were formed by the Mythic Dawn. Give or take some direction by Dagon, I suppose. So there would be Dawn agents all over the continent. Was that disproven?

And of course a wizard-did-it/ Oblivion works beyond mortal senses, but I assume resources and energy and logistics prevented Gates wherever they wanted. Until Camoran finally took the Amulet and went all-in.

Or something? bouncing ideas.

Gates required MD agents to create, as was established a few times in the game.

As far as gates opening elsewhere, there were a few in Black Marsh, but every time a gate popped up there, the argonians wrecked the living crap out of MD.

Also, Ald'Rhun was beaten to hell, even when they revived the Emperor Crab. And that place was where the heads of Great House Redoran lived. The only dunmer to somewhat effectively fight off the hordes of Oblivion was House Telvanni, and even then they only slowed down, not stem the tide of daedra popping out. Both of these were heard in rumors during the game, so agents of the Dawn were likely all over Tamriel.

It was when the barriers between Oblivion and Mundus everything when to hell and Dawn Agents were not required.

I'm still under the impression that cavalry would have helped. He didn't manage to do much damage to the countryside.