Snowygerry
Deity
What you just said made me realize something important.
There seems to be a point where, if you wish to succeed on the highest difficulties, Civilization 4 stops being a game and becomes a statistical min/max exercise. Cities are no longer seen as such, they become units of production potential within efficiency ranges. Buildings are defined as their opportunity cost (as you described forges), and everything is reduced to the hammer investment required.
It's still a game, just a different game, more like chess where some "moves" are simply "suboptimal" - that's why such emphasis is placed on where to settle the first city, for example.
A casual player may think "Screw that plains hill, I want to live on the coast..." and on the highest levels that may just be enough to lose the game right there

It need not be like that though, there's always a "comfort level" where the you can still afford long periods of peace, suboptimal decisons and then pull ahead again with a few successful wars.
Another fun "variant" is to simply turn all victory conditions off, including time, but not domination or conquest.
Now you're forced to wage war, but you can take all the time you want to go about it...
I don't want to ruin the enjoyment, but I would like to progress... tricky!
Another way to "progress" is ofcourse to try one of the many mods and scenarios - many offer increased challenge without necessarily upping the difficulty level.