There are two expansion packs for Civilization IV, Warlords and Beyond the Sword. Are these two separate expansion packs with unique features not found in either, or is the second (Beyond the Sword) a further expansion of the first (Warlords)? Put differently, if you're new to the game, is there any reason you'd still want to play Warlords (or plain Civilization IV for that matter) instead of going directly for Beyond the Sword?