Initially, I thought every faction was going to be very tough, but the reality is that it is not.
Spain was intially tough, but once I took out Portugal, the rest was rather easy. It was my first game so it took me a while to take care of the Emirate of Cordoba, or its bastarazied name THE MOORS, but once that happened..I took France, Western Africa and Italy without much problems.
I played as Scots. Conquered upwards and Ireland, then English down below and the rest is basically snowballed. Once you have a strong base of operations the game is literally continous.
I then played as the English, same thing as Scots but easier.
I played as Venetians, Sicily and Milan. Its basically the same thing, just spam Italian Spear Militia over and over. That 13 ARMOR gives it a HUGE edge over all units, except Mailed Mounted Knights, during the first 30 turns or so. They're simply unstopable, unless you have alot of more calvary and can outflank them, but its easier for them to re-produce cheap armies than it is for you to keep making more and more calvary.
Then I played as France, and I must say, it was my easiest game yet. It took me, literally, 15 turns to take all of the french rebels, both iberian rebel provinces, kick the english out and then charge them for a ceasefire (while destroying all of its armies in one single stroke) and building up two cities and two fortresses. So now I have infantry and calvary superior to everyone else, so whats the point? HRE is almost crumbling, Milan is down to half a stack and an empty city, Venice has some armies but they still cant match mine, Danes, Poland, Spain, Moors are all worthless. So thats a done deal. The 100 years war lasted 1 turn in fighting and 5 turns in siege attrition, and thats only because I wanted to see if I could actually persuade the English to give up their french position - which they didnt - hell they didnt even want a cease fire until I got the province from them..and they payed 10,000 gold too.
So, whats my problem? HRE. For starters:
It starts completely surrounded by rebel provinces. That means that every faction around it ex[ands into those provinces, and then THROUGH the empire, not anywhere else. Danes always go south, Polish always go West, French sometimes go East, Venetians and Milan always go north, as Sicilians. The only one who has yet to fight me is Hungary, and thats because they expand southwards.
That's still not the problem.
Its units are also meagre. Until you get cities and fortresses, you'll have to deal with larger odds, more armies, and a crippled economy with sub-par infantry in an infantry dominated sphere. You dont get good archers, infantry, or calvary. You're just average, fighting good Polish/French calvary and good Sicilian/Milanese/Venetian/Danish infantry.
And that's still not the problem.
Your economy is wrecked. Half your provinces are castles, and theyre all crappy, undeveloped castles. You're also at war with all of your neighbors, so that means that your trading isn't going to exactly "flow"...
And yet, that's not the biggest problem. The biggest problem is that the Alps and river crossings are so poorly placed, that it is nearly impossible to correctly support one end of your empire with another. You're under constant siege, by someone, so you need archers and spearmen like mad. you dont have the money to keep developing your economy, your units, or getting calvary. So you get an isolated section of your territory under siege by superior numbers, carrying a bunch of superior infantry, and guess what: YOU ALWAYS HAVE TO ****ING FIGHT THEM! Why? Because they took out oil, so rams ALWAYS, and I do mean ALWAYS, break through. How does a ram with peasants break through a fortress? Someone explain that one to me.
So, our overview of the HRE:
Surrounded by enemies, leaving it a question of WHEN and not IF they will attack.
Inferior infantry.
Crippled economy.
Due to crippled economy, you cant field enough calvary in order to beat back the endless waves of Milanese/Venetian italian spearmen, and/or saxon foot knights from sicily.
Map doesnt help you one bit with army movement.
Venice and Milan, are somehow, powerhouses. Economically and Militarily they simply own. I **** you not when I tell you that Im an experienced player and by turn 20 I am at war with no one and a 3 province faction has a bigger and better army, economy and production capacity than I have when I control 11 provinces -- most of which are cities (since I change them in order to make enough gold to support the ammounts of troops Im constantly cooking).
Therefore, I believe that the only way to be able to beat the AI in vh/vh as HRE is a pre-emptive attack on Milan, followed by attacks on Sicily and Venice ASAP. Once those three are out of the "picture" then you can deal with France, Poland and Danes with quite some ease if Im nto mistaken. The foremost problem here is that, aside from Sicily, Milan and Venice are able to re-fill their ranks at a small price and be back on the front before you're able to a) siege one of their cities, or b) re-group and re-train.