Current 1.75 Features:
New Game Options
Game Options are selected in the custom game screen.
Multiple Production
Multiple Production allows cities to produce more than one building or unit whenever possible. If a city has overflow production from the last item, it will be applied to the next item in the city queue. If the overflow production is enough to complete the next item, this process repeats. With large military cities, you can easily create multiple missiles or infantry units in one turn.
Multiple Research
The same concept as Multiple Production. If you have enough overflow beakers from your last research project to complete the next technology in the queue, you will research multiple technologies in one turn.
Usable Mountains
Usable Mountains transforms peaks from wasted space to strategic chokepoints and useful industrial locations. Resources can and will spawn on peaks, and units will be able to move over peaks after you have researched Mountaineering. After researching Algebra, cities can be constructed on peaks. Peaks now give 3 , but no food or commerce. Any improvements that can be built on hills can be built on peaks as well.
There is a new promotion, Mountaineering, available to Great General units. With it, all the units in the group will be able to cross over peaks without the aid of the Mountaineering tech. Peaks offer units a 75% defense bonus, the largest in the game.
Fixed Cultural Borders
Fixed Cultural Borders completely changes the way borders worked. Instead of Culturally-Driven borders, Military force and might dictate the spread of borders. With Fixed Borders, Units have new command "claim territory" that you can apply in an enemy territory. You claim it, next turn the plot is inside your borders. In order to recapture it, enemy must place his forces on that plot and do the same ("claim territory").
When capturing cities, you get all the territory that the city 'provides' to its original owner. So when you're at war with someone and capture all his cities, you will get ALL his territory, up to the last plot (there's only one exception to this rule - the tiles that are not 'provided' by cities, but claimed by force can remain in the old civ, but for sure they can't go to other neighboring civ that is neutral in war). In order to keep all this territory you need only the military force. No neighbor of his that is at peace with both sides can benefit from the war. If someone wants that territory – he must declare war and capture it or trade for it (city trading, like in normal game).
During a war, if a tile has more of your tiles surrounding it than the enemys, it is claimed automatically, if no enemy units are defending it. This makes the borders less shredded at the end of war.
If you are not using a civic that "forces" fixed borders, the game acts as before (i.e. the culture defines borders) and once a player has the necessary civics all of his territory becomes ‘fixed’, no one can take it by culture means anymore and his units get the new command to execute when in enemy territory (‘claim territory’

. The player still culturally expands his borders over unoccupied territory and over nations that don’t have fixed borders (think of the Roman Empire that enforced its own borders, but influenced the surrounding tribes that had no solid state formed).
Surround and Destroy
Surround and Destroy gives attack bonuses for surrounding enemy forces. The more units you have around and enemy, the larger the bonus, up to the maximum of 60%. Particularly useful for sieging medieval cities.
Advanced Diplomacy
Advanced Diplomacy vastly changes the way diplomacy works from standard Civilization. Many new trading options are added, each with their own benefits, and potential disadvantages.
Embassies
Nations can now establish embassies in other nations. Embassies improve diplomatic relations between countries, but be warned; revoking a foriegn embassies rights will have disastrous diplomatic repercussions with the other nation. Embassies also allow the trade of Military Units and Workers between countries. In addition, Embassies are now required to unlock higher diplomatic functions with other nations, namely, Open Borders Agreements and Defensive Pacts. Be warned, Embassies allow for enemy spies to more easily complete their missions in your country.
Right of Passage
When players research Writing, they can request to have a Right of Passage agreement between two civilizations. The agreement is similar to open borders, in that it allows foreign trade, but the agreement only allows for defensive units to pass through the nations. Offensive units will not be able to enter the territory.
Trading Military Units
Certain Military units, like siege machines can be traded, with Advanced Diplomacy. To be traded, each player must have an embassy in the other players nation. Each tradable unit will be marked in it's respective civilopedia entry with "This unit is tradable."
Trading Workers
Workers, can be traded, with Advanced Diplomacy. To be traded, each player must have an embassy in the other players nation
Buying and Selling of Corporations HQ's
Nations, if they happen to own a corporation Headquarter, can also sell it to other nations, where it will be automatically moved to one of their cities. If a nation does not own the particular Corporation Headquarter, they can offer to buy it from the owner.
Trading Contact with Other Civilizations
Nations can buy and sell contact with other civilizations that they have not yet met. It allows you to meet civilizations who you would not otherwise be able to trade with for many centuries.
Barbarian Generals
Barbarian Generals allows units to gain Great General experience from combat with Barbarians. It also allows barbarians to get experience as well, and they will be able to spawn their own Barbarian Great Generals.
Assimilation
When you capture an enemy city, it retains the personality of the former owner. The conquering player will be able to build the former owner's Unique Unit and Unique Buildings in that city instead of their own.
Ruthless AI
The ultimate challenge. Ruthless AI removes any human aspects from the AI, making them cold, calculating, and cunning. The AI will expand strategically into nearby targets, destroy threats, and use diplomacy to it's advantage. Watch small border conflicts flare up into International conflicts, with the entire world arrayed against you.
Great Commanders
Great Commanders are a new use of Great Generals. While players will still be able to attach Generals to units or settle them in cities, they will also be able to use the general unit in stacks, and have Great Generals receive XP from successful combat in nearby tiles. Great Generals will be able to earn promotions from this XP, and the promotions will boost the stats of all nearby units. Other promotions can extend the command range of the general (the radius in which the general's effects may be felt) or the amount of effects the promotions provide.
Realistic Culture Spread
Culture will spread in a more realistic fashion, more quickly over low lying lands, like plains and grasslands, and slower over heavily forested hills and peaks.
New Content
Guilds
Guilds are a medieval variant of corporations. There are twelve total guilds, part of three separate branches, the Servant Guilds, the Victualers Guilds, and the Craft Guilds. Each of the twelve guilds offers separate benefits to cities that incorporate them, be it health, happiness, production, science, or wealth. Spreading guilds is identical to spreading corporations, except it doesn't cost money. Guilds will, however, incur maintenance fees; the best way to alleviate these fees is to use the "Guilds" economic civic. Guilds are very useful for increasing city production or making your cities happy and healthy. Most guilds become obsolete in the Industrial Era.
Modern Corporations
Modern Corporations expands the amount of Corporations in the late game. Corporations that give increased military training and experience are useful for catch-up. Other corporations can give happiness, health, culture, science, commerce, or espionage against rivals.
Advanced Nukes
Advanced Nukes expands the nuclear arsenal available to players, allowing players to access advanced biological weapons, and nuclear weapons that can destroy entire continents. Chemical and Germ warfare can be used, to ruin even coastal land, and utterly annihilate a city to nothing. Advanced Nukes provides the finest and latest gear in the art of destruction.
Mastery Victory
Mastery Victory is the ultimate victory condition. Players must survive to the end of time, and be the most powerful player. Having cities with legendary culture, or constructing wonders adds to your final score. Spreading your religion to the most cities also boosts your score. The player with the highest score at the end of time wins.
Resource Depletion
Resource Depletion allows for heavily used non-renewable resources to deplete from the the game. Each turn, the game measure how much you are using a resource, and based on your usage, the chance of depletion changes. As you rely more and more on a particular resource, the odds of it depleting increase.
In Addition to this, mines without any resource also have a small chance of depleting. When a mine depletes, no mines can be built on top of the tile ever again. While this will rarely happen in the early game, by the late game, players will be forced to re-adjust their strategies if they relied too heavily on mines for their production.
To balance this, Players with Resource Depletion installed will also have an increased chance of discovering resources.