Right, first few levels, just do it as normal, killin people. Then,as soon as you have enough cash to buy the first thing (the one that allows you to convert people), buy it. for the next couple levels, go about killin people, and make sure you have a constant stream of people going into the castle, for conversion. When you have enough for the archery range, buy it, and click the archery range flag to put all the converted people in. play as normal, converting people to archers....until you have the money for the last thing (wizardy whatnot - make sure you have plenty cash left after buying it - i.e. buy it when you have 50, 000 (it costs 40k)) then convert people and kill people, and get loads of wizards. Dont bother with the other things (demolition and repair thing) and dont bother building castle up - well, maybe add a layer, if you want to.
The knack is to get as many wizards as you can, quickly. once you have 20, i think, you can use "d" to convert people, instead of killing them. Use this, where possible, and use "s" to nuke them, making sure noone gets to the castle. Build up wizards till you have like 70-80, then add a load more archers. once you get to the stage where youre doing well, just add wizards and archers - i ended up with 200 wizards and 165 archers in my game, and then you cant lose - literally...the archers will do most the work, and use the wizards to convert people where you can (and nuke if theres a load of people attacking.
once you have a dump load of money, get the repair thing, and get 20 or so of them people, may as well....
continue till you get bored!