The Slavs represent their namesake Central and Eastern European peoples that later become the founders of Poland, Russia, Czechia, Slovakia, Croatia, Serbia, Ukraine and many other (South-) Eastern European countries which use the Slavic languages. By the time frame of Ages of Empires II, the Slavic tribes had already split into the Western, Eastern, and Southern branches.
The Slavs civilization is mostly representative of the Eastern Slavic identity of the Kievan Rus', whose semi-autonomous principalities and city-states covered much of modern-day Ukraine, Belarus, and Western Russia. Politics of the time were dominated by Boyar Aristocracy, which was adapted into the Slavs' unique heavy cavalry unit.
Educated by the Byzantine Greeks, they adopted the Eastern Orthodox branch of Christianity as their religion, and to depict this, their Castle Age unique technology, Orthodoxy, gives Monks extra armor. Slavic armies are mainly composed of infantry, and to depict this, they have access to all infantry upgrades and their Imperial Age unique technology, Druzhina, gives infantry trample damage, making them powerful in groups and against clusters of units, and they also receive the Tracking technology for free.
Slavic lands in modern-day Ukraine were especially fertile and productive, later to be known as the Breadbasket of Europe, thus Slav farmers receive a bonus. The Slavs were highly influenced by the Byzantines and learned many techniques and technologies from them such as siege engines and military logistics, and to depict this, they have cheaper siege weapons and their team bonus makes military buildings provide +5 population.
"The icy plains of Eastern Europe have more under their thick crust of permafrost than meets the eye. After the dissolution of the Mongolian Golden Horde and the emerging of the Khanates, the eyes of the newly formed empires set their eyes upon the west. Thundering hooves of Kievan and Moscovite cavalry, escorted by large squadrons of men at arms terrify the established kingdoms on the horizon."
Despite being a Siege or Infantry civilization on paper, the Slavs are an offensive, economic civilization with powerful infantry and siege weapons. They can mount an offensive strategy using a jack of all trades strategy similar to the Byzantines, despite being slightly less defensive. They have a good economy with slightly faster farming, with the absence of Stone Shaft Mining and Guilds upgrade as the only drawback. Their Monks are tougher than most because of their first unique technology, Orthodoxy, which gives them +3/+3 armour. They have cheaper siege weapons due to their civilization bonus. They have powerful infantry units as they have all infantry upgrades and their second unique technology, Druzhina, gives infantry AOE damage, making them powerful in groups and against clusters of units. Their cavalry is somewhat lacking as they don't have the Paladin upgrade but they make up for this with their unique unit, the Boyar, a cavalry unit with heavy melee armour that can be used as a damage soaker for other units. While they have powerful infantry their archers are weak as they don't have the Arbalest and Bracer and are missing some other archer upgrades such as Thumb Ring. The Slavs also lack both Faith and Heresy, and due to the Slavs' dependence on siege weapons and expensive units like the Boyar, this makes the Slavs incredibly vulnerable to monk-heavy civilizations like the Spanish and the Aztecs, which have strong monks who are capable of converting Boyars which are likely to be converted as easily as Paladins or Persian War Elephants.
Alexander Nevsky - served as Prince of Novgorod (1236–52), Grand Prince of Kiev (1236–52) and Grand Prince of Vladimir (1252–63) during some of the most difficult times in Kievan Rus' history.
Boleslaw the Brave - was Duke of Poland from 992 to 1025, and the first King of Poland in 1025. As Boleslav IV, he was also Duke of Bohemia between 1002 and 1003.
Casimir II the Great - possibly refers to Casimir II the Just (1138–1194), Duke of Cracow and senior prince of Poland, or Casimir III the Great reigned as the King of Poland from 1333 to 1370.
Oleg the Seer - a Varangian prince (or konung) who ruled all or part of the Rus' people during the early 10th century.
Ottokar I - Duke of Bohemia periodically beginning in 1192, then acquired the title King of Bohemia, first in 1198 from Philip of Swabia, later in 1203 from Otto IV of Brunswick and in 1212 from Frederick. He was a member of the Přemyslid dynasty.
Sviatoslav the Brave - also spelled Svyatoslav was a Grand prince of Kiev famous for his persistent campaigns in the east and south, which precipitated the collapse of two great powers of Eastern Europe, Khazaria and the First Bulgarian Empire.
Vladimir the Great - was a prince of Novgorod, grand prince of Kiev, and ruler of Kievan Rus' from 980 to 1015.
Wenceslaus I - Saint Wenceslaus I, Duke of Bohemia (907–935 or 929), and subject of a Christmas carol.
Wenceslaus the One-Eyed - King of Bohemia from 1230 to 1253.
Yaroslav the Wise - thrice grand prince of Veliky Novgorod and Kiev, uniting the two principalities for a time under his rule.
Yuri II - the fourth Grand Prince of Vladimir (1212–1216, 1218–1238) who presided over Vladimir-Suzdal at the time of the Mongol invasion of Russia.
The Slavs are roughly named after the pre-divisional Eastern Slavic identity of the Kievan Rus'. As the TedED Video on the history of Russia best explains: they were a people "populated by Slavs, ruled by Vikings, taught by Greeks, and split by Mongols, would develop differences in society, culture, and language that remain to the present day."
The Slavs were the second Russian-type civilization to be featured in the Age of Empires series, preceded by the Russians in Age of Empires III.
Despite the fact that Castles are military buildings, they are not affected by the Slavic military building population bonus. This is most likely due to the fact that Castles already provide population.