If regicide is still in civ 4, I think that workers should be able to build Underground bunkers (Later in game) and just plain Bunkers (early in game). The underground bunkers would be deteced by stealth fighters and bombers, and by other units that can see invisible units. It would be like an invisible structure. The bunker should have a defense... depending on the age, how tuff. And you can add extra units do defend it. This way everyone wouldnt rush your capitol, and the Smaller civs that aren't doing so well can hide their king until they build a better army. I got this idea off of the history channel, It was on the war in Iraq.