I have yet to win on the 'toughest' lvl. I play mostly at 'tough' (I can beat the AI there
). I am however quite impressed by the AI, most of the time. 've played games at that level where my enemy didn't bluntly attack my nearest city as is the case by most other RTS's. Instead it just circled my strongpoint and took one of my lesser defended cities. Just like humans would do.
I mean you can't just sit there and wait until the AI bangs its head against your strongest defense. You have to invest in some sort of quick reaction force.
Of course you can try to rush the AI, but it isn't always easy meat. Some nations can counter that strat quite well. The nations I prefer aren't good at rushing BTW (British, Dutch and Russian)