Hellenes,

I would say that the multiplayer campaign would be far far easier to program in itself. But there are two major problems with it:

1: As Revenant69 said, you'd most likely still need to have a programmed opponent within the campaign mode (probably a lot easier to do than in battle mode, I'd imagine).

2: Most people tend to shy away from multiplayer campaigns, and it's hard to get a system that people like. My experiences of Civ3 were that different people liked different kinds of campaign style, hence why that game had so many different multiplayer modes. Each mode also had a number of different settings. So adding (and testing) multiple styles of campaign mode with multiple options each increases the challenge considerably. I'd also say that the generally slower pace of these does not appeal to the more casual gamer, at whom RTW was aimed - I think that may have played a big factor in their decision as to whether or not to include a multiplayer campaign option.