@Fearzone by turn 200 I'm lucky if I have two cities build, and I'm struggling to come up with materials to buy unit's, buildings, etc. How do you manage to launch an invasion force so fast? I'm not trying to be snarky, I seriously want to know as I view my slowness as being a weakness in my gameplay.
I usually a fairly huge kingdom by turn 200 and have knocked out anywhere from 1 to 3 AI by this point. They're especially easy to take down early in the game, as late-game they build fairly good units like catapult squads. I alwasy play on large maps.
Build nothing but pioneers & observers early on. Buy walking boots for all your heroes right away, and/or travelling cloak. None of them should be moving less than 3 mp a turn, and you should have at least some 5-movers. In newly settled towns buy caravans, that first diplomacy tech needs to be researched fairly soon. Start taking advantage of the halved cost for caravan routes right away. On large maps, I find caravan routes already set up .. use them to great effect. Once you conquer that first AI, you've typically acquired a well-built up city & start pumping out more caravans, pioneers, and observers, and eventually better combat units. Once you get squads start building them. Well built-up AI cities usually have a higher production rate, I've actually been able to build some 4-man squads on some AI cities in 1 turn. That's a great boon & start using it right away to great advantage. Make "time" your ally, think blitzkrieg not maginot line.
Anyone that moves 3 to 5 will move 6 to 10 on roads. That's an incredible boon, especially going for conquest on large maps on ridiculous setting as I do. Needless to say, make a beeline for military techs & market improvement so you can raise more money to support a larger military. Choose "increase gilder" each time a city levels-up, you don't need anything else. Make sure you also make a priority: crystal and shards in that order. Crystal are an incredible boon to building more effective military squads in mid-game, shards will also improve your spells, especially if fire shards & fire spells which are strong in attack.
Also, make sure you imbue as many guys as you can, keep your soverign fighting so he can level up more & imbue more champs. If you get to the point where you're wallowing in gilder, buying a longbow or two for some of your champs is a good idea. As far as spell tech, take fire-book and make a beeline for fire dart, fireball, fire-giant, fire swarm. You should have fire giant & whatever that demon spell is by the time you find your first AI, caste it & conquer. If you're very lucky you'll find that sand golem tablet. These monster armies will take down pretty much any AI in the early game before you've gotten squads, mid to late game they get eclipsed by archers & squads. Archer squads & horse archers should be your unit of choice in mid to late game, they're pretty much invincible in combo with strong spellcasters who pick up once your mana runs out.
Spend more your money on units less on buildings. Let the AI build up their cities for you .. when you conquer them you'll get a full plate & make up to some degree what you didn't build up in your own cities.
That's basically it, you should never loose this game even on ridiculous by following the above strategy. This game is a perfect example of Clausewitz's addage: "the best defense is a good offense". The AI is a pushover in the game-as-is. Hopefully 1.1 will improve the AI dramatically.