Lag only in Fort battles ?!?

[Nyogo]

Hi , my computer pass the minimum system reqirements
P 4 2.40 Ghz , 512 Mb Ram , and a video card Ati Radeon 9550 with just added new drivers from their site. I had no problems with installing the game
When I play on a plain terrain , even with many soldiers the game runs fine
but when there is a siege or defence of a fort , city and so on
it starts to lag and is imposible to play. The campaign map runs fine too
Also All video settings are set to minimum....
I will be glad if someone gives me a suggestion why it happens so and what can i do about this problem...
( I have no intention to buy video card or ram )

[DrIstvaan]

Then you're out of luck; your VGA is very old, and you have too little RAM. Sieges are very resource-demanding, building details can cause SEVERE lag.