Enthralling and Complex
It is in the advanced mechanics that Civilization IV truly shines as a game that is easy to learn but difficult to master. Starting on a Duel-sized planet with the 'Continents' map (one of several that have become the stock-in-trade of games from Age of Empires to Empire Earth), at the simplest level of difficulty, "Settler," I found the game thoroughly enjoyable and full of open-ended decisions that make every game absolutely unique.
The complexity of the technology tree in deciding what kind of civilization you want to become is also impressive, as it requires they player to sit and develop a strategy that is effective for the purpose of play. Sinking dozens of early-game turns into exploring and founding the various religions in the game was a fatal mistake when the civilization next to me - Genghis Khan - was ramping up the military tree to march through my cultural borders in a brazen act of war that decimated my economy.
The competing methods of play involved in any game of Civilization make it a game that can be played, saved and reloaded over many sittings. War is not a necessity to win - cultural or diplomatic victories can be just as sweet as demolishing Queen Victoria and laying waste to London and York. This fact alone means that those who have sought to create burgeoning cultural and trade empires in games like Empire Earth finally have an excellent outlet to exercise their dreams of a world filled with culture and commerce.
I highly recommend Civilization IV to anyone who has played the Age of Empires, Empire Earth or Command and Conquer series, as well as players just getting involved in RTS games. Civilization has something for everyone, from the casual day-gamer to the all-night, Jolt-and-Cheetos fueled endurance race of Marathon mode. The only roadblock is deciding who out of the plethora of world leaders to play as!