Most of these have already benn said, but anyway:
Early
- If I get blocked in by an AI
- If I find I don't have either Iron or Horses
- To get more land
Middle:
- When I get catapults
- If any neighbouring AI is weak
- As soon as I can build Cavalry
- To claim the rest of a medium sized continent to myself
Late:
- If for some reason I didn't get enough land to get some oil I'll grab the nearest patch
- When I finish researching Artillery and I can upgrade all my old siege weapons
- If I haven't guarenteed a win yet, either through having to finish off a domination, or by crippling all the AIs enough to make sure I can win the space race easily
Of course this is only based on the 5 games I've played so far, (1 Noble, 1 Prince, 2 Monarch, and a OCC Prince game, which has definately been a different and very interesting game), so I still haven't got rid of my Civ3 way of doing things yet
