I played this game about a year ago (1.3 I think). It was challenging, but not impossible against 3 non-allied CPUs, you just had to learn how to harvest and produce effectively until you could build up a massive force of robots (the highest unit at the time) and wipe each one out, one at a time. I think I had 6 castles by the end, and all had max gold/stone/wood.
I just played version 2.0.0 and have gotten hammered every time on 'Island Siege' (only map I've tried so far) against 3 non-allied CPUs. The AI attacks sooner and with a formidable force than before, which is good. But EVERY computer team attacks me and only me! No they're not allied, they attack each other, but only after they've slaughtered me. It might just be the 'Island Siege' map, I've only played that one because it has lots of gold at the start and I was determined to beat it.
I've tried several times and done it to about 80% of perfect. That is, I harvested gold and produced workers madly for the first few minutes, had 2 workers to get my barracks up ASAP, built a second barracks so I can produce faster, switched half my workers over to wood so I don't get any resource bottlenecks, etc. I was pretty much producing units constantly but I just couldn't hold off 3 teams.
My best effort was when I didn't bother with swordsmen or advanced swordsman, and just went straight to Horsemen (and built lots of archers too). After I held off the second raid (from all 3 teams, again!), I switched to making robots. I held off a 3rd raid quite easily and thought I was on top. By this time I had about 10 robots, 8 horsemen (20-odd had died), 15 archers (30-odd had died) and fully upgraded swords and shields, the only thing I hadn't done was build gyrocopters. I was annihilated by the 4th raid. When I looked at the final scores I had 172 kills and 104 deaths. The other teams had 65, 54 and 40 kills, only one team had more kills than deaths. I had more kills than all of them combined but I still lost.
Maybe it's just that map where I'm in the centre that makes the CPUs all come after me. It might be possible to win that map against 3 CPUs, but it's bloody hard.