The Triumphal Arch is a buff building -- raising its level provides rally/reinforcement and battlefield buffs. Buildable at Keep 38, and the Keep cannot reach 39 unless the Arch is built (Lv1). The Art Hall must be raised first; the Arch cannot exceed Lv5 unless Ideal Land is Lv7+ (Lv6 needs Ideal Land 7, Lv7 needs Ideal Land 8). Buffs by level (Lv1-7): Rally Capacity +3%->+21%; Reinforcement Capacity +3%->+21%; Rally Capacity in Battlefield +10%->+70%; Reinforcement Capacity in Battlefield +10%->+60%; Troop Attack/Defense/HP in Battlefield from Lv3 (+10%) to +50% at Lv7. All menus (Glory Achievement, Monarch Battle Logs) usable during upgrade. Upgrade requirements are achievement-gated: Lv1 needs Art Hall 38; higher levels need Gaugamela/Constantinople score thresholds, All-Star placement, consecutive wins, and (Lv6-7) Ideal Land 7/8 plus server-war personal-point milestones. Resource costs are very large. For a buff-automation bot, this is a high-end rally/reinforcement and battlefield-combat buff source gated behind Keep 38, Art Hall and Ideal Land; low priority for economy-first play.
Original research source: Evony Guide Wiki