I buy units (especially settlers) early on when possible. Be sure to step buy the units though. There is a post here showing what step buying is all about. Anybody know exactly where it is?
If your opponent is working on Mich's, or you may even think they might be close to getting to it, buy it if you can. In a multi-player game, it usually makes the difference between winning and losing.
Throw as many caravans as you can into the city, or even disband units you no longer need (or can do without for now) into the cities production. Wonders cost 4 gold per shield. Another trick I use is to rush buy caravans. They cost less than a Wonder, but can be used 100% for building wonders.
As far as sepnding limits my general rule of thumb is the divide the $ needed to rush build by the number of turns left. If it is 8 or less I usually buy it. Also remember, you don't have to buy the whole item. Change the production to a lesser item that costs less, buy that item, then change the production to the bigger item and you are that many turns closer to finishing.
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Thanks,
Ray
[This message has been edited by Duke of Marlbrough (edited June 26, 2001).]