The Art Hall gives rewards and buffs as you collect art and generals; you must raise its level to receive the rewards. Buildable at Keep 11. It must be raised first to raise the Keep to 28 or 33, or to raise the Triumphal Arch. Its four menus: General Hall (rewards for collecting generals), Exhibition Hall (rewards for collecting Artwork), Art Treasure (check/upgrade Art-Treasure buffs), Civilization Cards (obtain/exchange/compose cards, view card buffs). Milestones tie rewards to level: Lv11-25 progressively unlock Exhibition Hall rewards (Lv11 Knight Equipment Hall, Lv13 Oil Painting, Lv15 Stringed Instrument, Lv16 Jewelry, up to Lv25 Mask Hall); Lv12 plus Lv27-35 unlock General Hall rewards (through Lv35 Legendary Hero, Invincible Ares). Keep-gated (Keep 11 through early levels then 1:1), speed to +610%, Power ~48M at Lv50. Artwork Fragment Chests fill the Exhibition Halls; each hall's three tiers require 20/60/200 items and pay Gems, Stamina, speedups, Research Stones and Adv City Teleporters (all free once artworks are collected). For a buff/collection bot, Art Hall level unlocks reward tiers from accumulated generals and artworks and gates Art-Treasure buffs and the Mona Lisa gathering buff.
Original research source: Evony Guide Wiki