The concept of a boxed expansion pack to a popular game has largely been abandoned for incremental DLC, but Firaxis aimed for Civ V: Gods & Kings to expand the original game in nearly every way. More than just additional civs, units and technologies, Gods & Kings brings whole new systems to Civ V's turn-based design which force you to rethink old strategies. Once you play it, vanilla Civ V just won't feel as complete.
The mandate to spread your religion returns from Civ IV, finally allowing some non-combat options for the peace-minded players out there. You accumulate faith points through building shrines and spend them on founding first a pantheon, then on a full-on religion, then on Great Prophets and missionaries to spread the Word. The late game of Civ could sometimes be boring when going for a culture or diplomatic victory, but the managing of these new units peacefully competing with other religions can be just as fun as plotting a military invasion. Designing your own religion through choosing the beliefs to fit the needs of your civ, or your particular playstyle, was a great call - a city following your religion could make a certain building more powerful, while each converted city can provide +1 happiness to the founding civ. The number of available religions is always fewer than the number of rival civs in the game, so you want to found one early or be forced to adopt a rivals. You can even deviate from the real-world names provided such as Confucianism or Christiantiy and rename the religion - just please resist the urge to name your religion Bob's Tantric Sex Doctrine.
The addition of espionage attempts to make diplomacy between civs more nuanced. You get your first spy in the renaissance era, and an additional one every era after, which adds further complexity to the end game. Through a menu, you can move these spies to any city on the map. Put a spy in an independent city state and she will rig the election, making the city your ally. If a spy is in an enemy civ's city , he can steal technology after several turns, as well as dig up plots the leader is cooking up. I was excited by this feature at first, but in practice your spy will just spam you with the schizophrenic plans of the AI leaders . "The Mayan Pacal is plotting against Gustavus Adolphus of Sweden." "Attila the Hun is building up to sneak attack Austria." You can choose to share that info with the intended victim to curry favor, but the frequent reports of the AI's shifting plans lose value fast. The usefulness of what spies can do doesn't really make up for the time needed to manage them. Why can't they poison the water supply? Or plant a nuclear device? Or drink a martini shaken but not stirred?
If you're a warmonger at heart - Gandhi, I'm looking your direction - the way you conduct war is more complex in this expansion. The single unit per hex combat in Civ V rewarded mixed unit tactics on the land but the strategy for navies was simply to build the most powerful ships available. Gods & Kings marks some ships as "melee", meaning they can take over cities on their own instead of merely supporting land forces. With the addition of new ships like the ranged-attack-only galleas giving you multiple naval build choices in every era, a strong navy of mixed units can take over coastal cities easily. Once you have a few Great Admirals - equivalent to Great Generals who can instantly repair naval units - a civ founding a city on the coast is a downright risk. Navies are a bit too powerful now - on archipelago maps there's almost no reason to build land units - but sea-centric civs like the Byzantines or Carthage dominating the waves satisfies the history buff in me.
The roster of units available to you has expanded considerably. The early modern era now has a slew of World War I-styled units like the landship (first tank), the triplane and Great War bomber. These slightly weaker units even out the technology tree so that racing to gain flight, say, doesn't immediately give you a huge edge in warfare. I was worried that just adding units would dilute their importance, but in play the variety of units on the battlefield feels organic. I could see the slightly outmoded Great War infantry of Haile Selassie from Ethiopia dropping the new atomic bomb unit - delivered by a B-29 - just to kill the superior tanks and infantry of the Netherlands.
Perhaps the most welcome changes to Civ V are under the hood. The underlying code has been sped up to reduce the time calculating between turns, but there's still a longer delay than I'd like. The AI can handle land tactics well now, using ranged attacks and flanking maneuvers, and defending cities will concentrate fire on a single attacker to eliminate it. How the AI handles navies may need some tweaking, but your computer opponents generally know that a big navy will help them defend coastal cities.
More is not always better, but, in the case of Gods & Kings, the sheer amount of new content and concepts won me over. The strategic and tactical options available to the player increased so much that the possibility for the game to surprise and excite me has multiplied.
Bottom Line: An excellent collection of new content and concepts for one of the world's favorite strategy games, once you play Gods & Kings you won't want to go back.
Recommendation: Pick it up if you have Civ V, or even if you don't because you gave it a pass over Civ IV. It may change your mind.