FreeCiv is an open-source game based on the P.C. game Civilization of the early 1990s. You can complicate it as much as you like, since the program is highly-customizable. But the concept is simple. You need to expand your empire geographically as far as it will go, and either build a spaceship to land on another planet, or just methodically kill the other non-allied nations.

The layout will vary based on your version of the game, and your format (mobile, desktop, PC, Mac, Android)

I. Units

 a. Primitive

  Settlers (city builders)
  Workers (builders of roads, railroads, irrigation, farms, mines, derricks)
  Warriors, Horsemen, Archers, Phalanx (offense and defense)
  Explorer (explores new territory)

 b. Iron age

  Trireme (boat, can hold three units)
  Diplomat (for embassies, bribes, rebellions, sabotage)
  Chariot, Catapult, Legion (offense and defense)

 c. Advanced iron age
  Caravel (replaces Trireme)
  Pikemen, Knights (replace warriors, horsemen, archers, phalanx)
  Caravan (trade)

 d. Gunpowder and steam power

  Engineers (replaces workers)
  Musketeers (replaces legion, pikemen)
  Dragoons (replaces knights)
  Cannon (replaces catapult) 
  Galleon (replaces caravel)
  Frigate (military ship)
  Ironclad (replaces Frigate)
 e. Industrial age
  Transport (replaces galleon)
  Freight (replaces caravan)
  Riflemen (replaces musketeers)
  Destroyer (replaces ironclad)
  Cavalry (replaces dragoons)
  Alpine troops (offense and defense)
  Spy (replaces diplomat)

 f. Technology age

  Partisan (replaces explorer, is armed)
  Artillery (replaces cannon)
  Submarine, Cruiser, Battleship (offense and defense)
  Marines (offense and defense, can fight from ship)
  Fighter plane (first air assault)

 g. Computer age

  Armor (replaces cavalry)
  Howitzer (replaces artillery)
  Mechanized Infantry (offense and defense)
  Paratroopers (dropped long-range from airbases and friendly cities)
  Carrier (for aircraft transport)
  AEGIS Cruiser (replaces cruiser)
  Helicopter (can take over cities)
  Bomber, AWACS (air assault)
  Cruise missile
  Nuclear missile

 h. Space age

  Stealth bomber (replaces bomber)
  Stealth fighter (replaces fighter)
  Space ship (for your future home in Alpha Centauri system)

II. Terrain and Special tiles

 a. Glacier (ivory tusks and oil)
 b. Tundra (bear furs and moose)
 c. Desert (oil and oasis)
 d. Plains (buffalo and wheat)
 e. Grassland (general resources)
 f. Forest (pheasant and silk)
 g. Jungle (fruit and precious gems)
 h. Swamp (spice and peat)
 i. Hills (wine and coal)
 j. Mountains (iron and gold)
 k. Lake or ocean (fish and whales)
 l. Deep ocean (none)

III. Terrain improvements

 a. Roads (2 to 6 turns depending on terrain)
 b. Bridges (for land masses with a river)
 c. Railroad (replaces roads)
 d. Irrigation (tundra, desert, plains, grassland, hills, 2 turns)
 e. Farm (replaces irrigation)
 f. Mine (hills, mountains, 5 turns)
 g. Oil derrick (glacier, desert, 5 turns)
 h. Fortress and air field (land units)

IV. Changing terrains (5 to 36 turns)

 a. Glacier becomes tundra
 b. Tundra becomes desert
 c. Desert becomes plains
 d. Mountains become hills
 e. Hills become plains
 f. Plains become grassland or forest
 g. Grassland becomes hills or forest
 h. Forest becomes plains, grassland or swamp
 i. Jungle becomes plains, grassland or forest
 j. Swamp becomes forest, grassland or lake/ocean
 k. Lake/ocean becomes swamp

V. Buildings

 Palace (your capital city when you start)
 Aqueduct and Sewer System (city growth, removes population cap)
 Granary, Harbor and Supermarket (food growth and food storage)
 Temple, Cathedral, Colosseum (makes citizens happy by distraction)
 Courthouse, Police Station (reduces corruption, helps maintain order)
 Marketplace, Bank, Stock Exchange (increases profits)
 Barracks (I, II, III), City Walls, Coastal Defense, SAM, SDI (defense)
 Library, University, Research Lab (speeds up technology advancements)
 Super highways (trade improvement)
 Airport (for air units)
 Port facility (for sea units)
 Factory, Manufacturing plant, Offshore Platform (for faster production)
 Power Plant, Hydro Plant, Nuclear Plant, Solar Plant (improves production, reduces pollution)
 Recycling Plant, Mass Transit (reduction of pollution)
 Space structural, component and module (spaceship parts)

VI. Wonders of the World
 a. Early 

  Colossus (extra trade resource around city, obsolete by flight)
  Copernicus Observatory (improves science production in city)
  Great Library (Gain one advance each turn known by two other nations, obsolete by electricity)
  Great Wall (acts as city wall in all cities, obsolete by metallurgy)
  Hanging Gardens (happiness improves, obsolete by railroad)
  King Richard's Crusade (adds resources around city, obsolete by robotics)
  Leonardo's Workshop (upgrades one obsolete unit per turn, obsolete by automobile)
  Lighthouse (adds sea unit movement, veteran status, obsolete by magnetism)
  Magellan's Expedition (adds sea unit movement)
  Marco Polo's Embassy (adds embassies in other nations, obsolete by communism)
  Oracle (happiness improves in temple cities, obsolete by theology)
  Pyramids (improves food storage efficiency)
  Sun Tzu's War Academy (Land units start as veterans, obsolete by mobile warfare)

 b. Modern

  Adam Smith's Trading Co. (Eliminates upkeep cost of some improvements)
  Darwin's Voyage (Gain two immediate technologies)
  Eiffel Tower (Reputation and goodwill among nations recovers more quickly)
  Isaac Newton's College (improves science production in university cities)
  J.S. Bach's Cathedral (improves happiness)
  Michaelangelo's Chapel (acts as cathedral in all cities)
  Shakespeare's Theatre (makes all citizens content in city)
  Statue of Liberty (opens all forms of government, eliminates anarchy period)
  United Nations (units regain extra hit points each turn)
  Women's Suffrage (acts as police station in all cities)

 c. High-tech
  Apollo Program (allows production of spacecraft, map fully visible)
  Cure For Cancer (improves happiness in all cities)
  Hoover Dam (acts as hydro plant in all cities)
  Manhattan Project (allows production of nuclear bombs)
  SETI Program (increases science production in research lab cities)

In the game itself, you can look at the research tree to see how one advancement leads to another. It's a lot of information to digest. Practice the gameplay on the novice and easy levels until you get the hang of everything you have to do in one turn, everything from producing units, managing money, improving land, exploration and self-defense. For my strategy, assume gameplay on the normal level.

First, I change my research tree to "Pottery" for the first advancement, and "Seafearing" for technological goal. Pottery allows you to build workers (hugely important) and granaries. Once you achieve pottery, your nation will automatically follow the research tree to seafearing, which will allow you to build a trireme (boat) and construct a harbor (extra food).

You start the game with two settlers, two workers and an explorer. I like to establish the first city immediately if possible, and the second city within a few turns. Put both on water, so you can explore other land masses as soon as possible. Your explorer should be scouting the land. Your workers should be irrigating and building roads and mines. I also set "new citizens" to scientists in each city's production profile. Expansion is critical. You must beat the other nations to uninhabited land, and the huts quickly. I accept all cease fire and peace offers from other nations early in the game. Nothing slows your progress like fighting wars with sticks and stones and choosing military advances over scientific ones. Choose your alliances carefully. If you have more than one nation wanting an alliance, make sure they're friendly with each other before choosing both. visiting a hut can speed up your progress. Most common in the huts are small sums of gold and military units. Less common are scientific advances, friendly cities and settlers. Occasionally huts are hostile, unleashing barbarians or simply killing your unit.

When choosing a city site, choose it with production and food in mind, and try not to overlap your other cities too much. Ample food will allow your city to grow. Production elements will allow you to produce things more quickly, especially whn you reach the advanced arms race and the space race. Coal, iron and oil are the biggest production boosters, even more so with a mine or derrick on the land. Peat, pheasant, silk, gold, buffalo, moose and whales offer minor production boosts. Fish, whales, desert oasis, wheat, buffalo, pheasant, fruit, spice, moose and bear furs boost food. Wine and gems are for trade value. Finding a piece of land with coal, iron or oil available is the most valuable to me.

All nations start with a despotic government. Ideally you want to go from despotism to republic to democracy. Monarchy is for warlords without affecting trade. Communism is for war-mongerers without affecting science.

My first research profile: Pottery, Alphabet, Maps, Seafearing . This allows me to build workers, granaries, boat(s) and harbors. From there, I choose Bronze Working (for phalanx), Masonry (for walls), Ceremonial Burial (for temples) and then the research goal for the Republic. Once I achieve republic and change the form of government, I select the research tree for Theory of Gravity. After that, I choose the path to Economics, then the path to Democracy. After changing governments again to democracy, I set myself on the paths to steam engine, steel, automobile and robotics, with sanitation, refrigeration, amphibious warfare and recycling at points in between. From there it's an easy step to the space-related and stealth-related technologies. The point is to become so productive, and so rich, that no other nation can touch you. Your empire should be able to produce lots of howitzers, battleships and stealth bombers, and when the time comes, spaceship parts.

Other nations dive into the building of the wonders of the world quickly, at a rate faster than you can achieve with my strategy. Don't fret. Most of them will be useless through advancements anyway. While they waste money on things, you're building your bank account and expanding to other lands. However, I like to pursue the Pyramids (food efficiency) early. I always try for the Great Library, but 95% of the time with my research plan, someone else beats me to it. Copernicus' Observatory and King Richard's Crusade are nice, but don't knock yourself out to get them. Magellan's Expedition takes a long time to build, but worth it for your naval battles.

The middle/modern wonders are the most important. With Gravity, I can start right away on Isaac Newton's College, which also takes a long time, but will jump-start your research speed. As soon as I research economics, I set forth on building Adam Smith's Trading Company, which will save me money on city upkeep in the long run. Eiffel Tower is quick to build and will help with international relations. I like to have Shakespeare's Theatre in a city where I plan to build stealth bombers in the future. Get Darwin's Voyage if possible, since that gives you two immediate advances. I generally ignore the Bach Cathedral and Michaelangelo's Chapel, and most of the time, Statue of Liberty. Women's Suffrage and United Nations are important if you plan to build a lot of military units, but prioritize the others first.

By the time you're in the high-tech part of the research tree, you should have no problem building the Hoover Dam, the Manhattan Project and the Apollo Program. Someone usually beats me to Cure For Cancer. I ignore SETI since I've already reached the end of the science tree anyway.

As mentioned earlier, an alliance is important. Your ally or allies can build the war machine and you can become a production and science dynamo. With a buddy to handle the enemies, I've been able to build the Apollo Program in the 1700s, construction the spacecraft by the 1820s and win the game by landing on another planet in the 1840s. But with advancing technology comes envy. When you begin building space parts, your allies or anyone with an armistice with you becomes suspicious. They stomp and whine about your attempt to dominate space. That's why you need to balance the production of space parts with the production of howitzers, battleships and bombers. On the normal level, when you launch your spacecraft (15 years to victory from that point), all alliances dissolve into armistices, and all other lower accords dissolve into war. Seven years after launch, your old allies will officially declare war.

VII. Events and other quirks

 a. Pirates (prepare to defend them, but it should not be too hard)
 b. Natural disasters (will destroy a building or reduce population periodically)
 c. Global warming (pollution will cause it, but it's usually your opponents' fault)
 d. Nuclear winter (two nuclear bomb detonations will cause it eventually)
 e. Deploying a worker or engineer draws your enemies' military like a magnet. It's frustrating.
 f. If you're the first to research philosophy, you get the next advancement immediately (usually university for me)
 g. My laptop gets too hot when playing this game, so I can't play for too long.