Okay, I'm assuming (from memory.. it's been ages - no pun intended
) that your building up in the northern corner is on the same island as the Jin.. that is definitely the best way to go about it - staying on the mainland makes things really really hard!
Anyway, I guess you probably just need to advance to Imperial quicker, and probably in this case, don't worry about getting trebuchets too much - wonders are pretty weak and getting more cavaliers is the better bet - then if you can, just wait until they come out of the gate and go in while it's open.. maybe get a few pikemen to counter the cavaliers they have (I think), and put up a castle near that top gate to take care of defence - get murder holes for it probably.
About 20 cavaliers should do the job fine then, after which you'll have plenty of time to build up to hit the other sides.
Oh well, that was fun - I haven't played the game at all in a long time.. only ever did on my pc, but funnily enough I've barely used that since having an iBook
. In any case, I might just be feeling like playing it again soon.