With the Teutons and Goths, the Vikings are one of the civilizations belonging to the Eastern and Norther European culture. Their major advantage is their stronger infantry and great naval technologies. All of their warships have 20% training discount. All of their infantry units enjoy 10% more HP at Feudal Age, 15% in Castle Age, 20% in Imperial Age. They are the only civilization which have two unique units (except that in the expansion set the Koreans and the Spanish also have two unique units). Their unique units are the Longboat, a very powerful warship, and the Berserk, a very power infantry unit who can heal himself gradually. They also get the wheel cart and wheel barrow free of charge when you progress to the next age.
Guide for Using the Vikings
Only play Vikings in water maps (ie, Highlands, Black Sea, Islands, etc.). In the Dark Age, Vikings excel because they get wheelbarrow and Handcart free. Try to build a lot of docks, maybe 2-4. Keep on attacking and build a castle so you can get longboats. Use longboats to not allow the enemies to attack you. In the Imperial Age, get Berserkers, Champions, arbalests, cavaliers, a villager, pikemenm and trebuchets. Let the trebs take out walls and towers. Your infantry will beat most others. Your villager can build some military buildings to send out troops in the enemies base. Charge into the enemies village and take over!