Since back then (AOE2) I liked one ally and four enemies. Makes every game different. And teaming the enemies up boosts the difficulty.   Did you somewhere establish what "non-cheating" is? I consider "all seeing" cheating, so... (Not complaining, just saying.)

The AI sends its only hero to die in its first attack. The hero dies, so the AI instantly lose. Make the hero stay in home base, near the Civic Center. I can see that the "retreat" trigger activates when the hero is at low health, but it doesn't work very well on slow infantry and Elephant heroes.

I partially agree, but you don't have to attack the fortified positions to be ahead. In fact, you should ignore the buildings and go after the enemy's gatherers. In multiplayer games, people will resign once they see you are far ahead. This is possible even in P1. 

It's a mod with better AI. Check the mods in the Mod dowloader. You might first try winning against a unmodded Medium AI, though.  It's the highest non-cheating difficulty that still can pose a good challenge if you procrastinate and let it build up.

I don't remember if it has already been suggested, but what would really better the game would be secondary objectives, and especially in P2. I mean, it's nearly impossible to attack an enemy fortified area before siege units in P3, which means the gameplay lacks diversity. We'd need things to fight for in P1 (like treasures/relics, but ones which would be revealed at the beginning of the game, to remove the randomness of getting there first if you don't know the map already). Note that many maps have Gaia buildings, but it's never (AFAIK) explained whether they have their own roots (some have, so are nice to capture early) or not (most of them don't, which makes them nearly useless to capture early). A few nice-to-have-but-not-totally-unbalancing buildings with their own root territory clearly established, and revealed at the beginning of the game, could be nice. Like towers, houses, markets, storehouses, farms... One map at least have temples with their own roots, it's a bit strong imho but why not. Not necessarily all treasures and relics and buildings-with-their-own-roots need to be revealed at the beginning of the game, it's fine to reward exploration, but a few important ones should. And all buildings should make clearly visible if they have their own root territory ! Of course those revealed at start should be at a minimum distance from the players' starting position (basically no treasure should be closer to one player than to at least another one, and other rewards should still be relatively far away, so that a player sending troops to it can get there before the closest player has finished to capture it if he did send only one citizen soldier).   And in P2, we should have things that resist attacks by non-siege units (like garrisoned towers) and simple, unprotected battering rams that can take them down (but slowly, and also are vulnerable to counter-attacks : basically no armor, even against pierce). Not sure what the equilibrium should be concerning resource gathering : we'd want something that can be built in the open (to serve as a secondary objective for the enemy), that is resistant to non-siege units once garrisoned but can be taken down by these P2 rams. But also not something that makes it too easy to build outside territory, so as not to compete unfairly with the Kleroukia or Town Center. Maybe something that take nearly as much time to build than a Kleroukia, doesn't cost metal and costs only 50 or 100 Stone ? With less hit points, less garrison, and way less territory around it ?

What is it ? I wasn't aware of it... Does it cheat ? As written above, I don't like that.

Thanks. I wasn't aware of difficulty levels for the AI. I really dislike cheating AIs, though. What is the highest level where it doesn't cheat ? Note also that I'm absolutely not asking for advice on how to play against AI. I like discovering myself what works, it's the most enjoyable part of the game.   My question was about how many AI opponent to choose (eventually, how many allies too - allies are good for trade, but it's quite random how long they survive, which is something I don't really like). And, for high level of opponents, what level of cheating (additionnal resources mostly) is acceptable and makes for a game that is both fun and challenging.