Save Resources: When you are allies with others, do not build a market to avoid giving them resources.
Explore enemy territory without being attacked: Advance to the Feudal age, build a Market, then create a Trade Cart. Use the Trade Cart to safely explore your opponent's territory. If your opponent is neutral, they will not harm the Trade Cart. Another trick is to create several Trade Carts and place them evenly across your opponent's area to see everything they are doing, without having to spend several thousand gold to research Spies/Treason.
Build Faster: Continuously select the icon next to the percentage.
Barbarians: Play in deathmatch mode and choose the Vikings. Instead of having Berserkers, they will have Barbarians.