The Arsenal upgrades already-created troops -- it enables troop-tier upgrades (converting lower-tier troops into higher tiers rather than retraining), with a dedicated Troop Upgrade Guide linked. Buildable at Keep 27. Critically, the Arsenal is capped at Level 1 -- no further upgrades exist. Its single Lv1 row costs 58,000,000 Food / 74,000,000 Lumber / 76,000,000 Stone / 74,000,000 Ore, requires Keep 27, example build time ~75d 21:19 at 0% speed, Power 10. Because it has one level, there are no milestone unlocks, no per-level scaling and no duty officer on this page. Its role is narrow but important: it is the gate that makes troop upgrades possible once you reach Keep 27. The page does not state whether the upgrade function is disabled while the Arsenal itself builds, and offers no other economy/buff effects. For a bot, build the Arsenal once at Keep 27 to enable troop upgrades, then it needs no further attention.
Original research source: Evony Guide Wiki