The faction tree (Faction Trees) shows you a compact overview of all units of a faction. In the left column you can select the different factions. Above the units shown, there are also various premium units that you can theoretically buy for real money. These premium units grant you more free experience beyond the premium time. Some of these units also allow you to immediately unlock a unit of the same class in the normal faction tree (see 2nd photo). In order to be able to play some units, such as the Chinese Crossbowmen, you must first play the Swordsmen up to Tier III (3). Some units also have abbreviations. For example, from the "Wu Marksman" tier (VII), I can also unlock the "Armoured Repeating Crossbows" (VII). With clever planning you can skip some units.