The Barracks is the ground-troop training building; higher level unlocks higher ground tiers plus training capacity/speed. Builds Lv1-50, each level requiring the matching Keep level; from Lv26 also Farm (and Shrine at Lv27/30/32/35), Lv40+ also Triumphal Arch 2 plus a Military Academy Ground Formation research. Troop-tier unlocks by level: T2@6, T3@9, T4@11, T5@14, T6@16, T7@19, T8@21, T9@23, T10@25, T11@27, T12@30, T13@32, T14@35, T15@40, T16@45, T17@50 (highest). Blazon at Lv8. Key milestone: Duty Officer at Lv25 (same level as T10; increases Ground Training Speed and Capacity; ideal Abd al-Rahman I, Liu Bei, Jiang Ziya). Cost scales steeply: Lv1 1,000 food/1,000 lumber; Lv25 ~11M/14M/15M/14M; Lv50 ~119B/152B/156B/152B. Speed +0% through Lv17, +30% at Lv18 to +610% at Lv50. For a bot, treat Barracks level as the gate for both ground-troop tier availability and ground training throughput; note research/generals/sub-cities cut training TIME, not per-troop resource cost. For a count/wall goal, low-tier ground (T1 no stone, T2) is the cheap meat-shield; high tiers (T12-14) are the real combat strength.
Original research source: Evony Guide Wiki