In a number of games we played, we managed to claw our way into the lead but discovered that holding onto it was another issue altogether. Civ IV calculates a running score for each civilization, and this is based on a number of details, such as civilization size, technology, and military power. We discovered that if you leave your civilization lightly defended or equipped with obsolete units, other nations will take advantage of the fact. We tried to play defensive games, where we "turtled" within our borders and built an advanced civilization. Still, we discovered that we needed to maintain a formidable military, if for no other reason than to deter our neighbors from attacking us. This meant upgrading our defenses with walls and castles, as well as upgrading military units whenever a new technology made them obsolete. For example, archers make way for longbows, which make way for muskets, which make way for infantry, and so on. You can upgrade existing military units for a cost in gold, or you can simply construct new ones. It's also important to make sure to build barracks, as they grant new military units an experience level, which allows you to upgrade units when they're created. For example, you can promote a unit by making it a more effective defender, a more powerful attacker, or bestow upon it some other useful trait, such as the ability to heal faster. And, of course, the best defense is a good offense, so you'll also need to maintain mobile and rapid units, such as war elephants, cavalry, and tanks, so you can take the fight to the enemy.