The Classic tech tree is the one where extensive research into new technologies must be conducted parallel to the progression of a game. Some real-time strategy (RTS) games as well as most turn-based strategy (TBS) games employ this type of tech tree. In these games, players typically have a 'command center', 'unit-training-facility' and 'research facility' at their disposal from the start, allowing them to both start research into more advanced technologies, or engaging in combat with enemies using the basic units (Warzone 2100, Mega Lo Mania, Master of Orion series, Civilization series, Space Empires series).