Every map has ancient and natural wonders that will boost your faction. Every map also has Atlantis hidden somewhere in the ocean. Finding it will provide finished technology free of charge. It can only be found with an open-water sea vessel (for example, a Galleon). Focus on naval technology so this can be done, as well as get a benefit over enemies that are more land-oriented; many other wonders are located on islands. Use Galleons or the Lighthouse of Alexandria to reach them.
Battle
Develop the Scouting skill to learn the strength of enemy cities. Rely on siege weapons such as catapults to weaken enemy defenses before engaging your full army. Combine a ranged unit, for example an Archer, with the wall structure provided by Masonry technology. This will let you damage enemies from your city at a distance and survive sieges.
Defense
When your civilization reaches a large size, it will be difficult to defend all cities without losing tech advances and city improvements. At this point, only defend the borders well. The cities in the middle of your civilization can be kept very light on troops.
Spying
Focus one city to produce one spy for each capital city of your enemies early. Use them to constantly interfere with your opponent's production speed. Although you will have less production from that city, you will have the benefit of having your opponents unable to catch up with your faster development. Russia's Catherine is well suited to this task because of her 50% bonus to spy production.
Various Tips and Tricks
America and China are two of the most powerful in the game.
No matter if you play an aggressive or a defensive game you need a solid defense because you’ll be declared war upon a lot.
Make sure you have walls and archers early on to defendi your cities, and few offensive units to destroy the enemy if need be.
Do not trust your neighbouring civilization.
Placeing a warrior on a hight point, hills or mountains nearby, will get you scout and you'll “see” better what troops are incoming and you can prepare to greet them.
When your civilization become very large it's far better to only defend the borders than individual cities. A solid defense and offense stacked around your border so that the cities can be kept very light on troops.
Try to be the first to get access to the Galleon (or the Lighthouse of Alexandria) so that you can explore the ocean. The goal is to be the first to discover Atlantis for the tech boost.
Plan your science development because getting all the techs in order is not the best idea.
Expand quickly if you want to make sure that you get a great spot and no other civilization take it. The sooner you build your second city, the easier for you to progress faster.