The Arsenal building (requires Keep 27) lets players upgrade existing troops one tier at a time: T10->11->12->13->14; troops below T9 cannot be upgraded at all. Requirements: the destination-tier facility (Barracks/Archer Camp/Stables/Workshop for Ground/Ranged/Mounted/Siege respectively) must already be built to the target level; troops must be inside the city (not gathering, hunting, rallying, or ghosting) and not wounded/in hospital; upgrades take time (speed-up-able with gems) and temporarily reduce monarch power by the upgraded troops' value until completion. Upgrading is NOT counted as 'Troop Training' for activity/event score purposes (except if completed on a 'Power Increase' event day). Economically, building a new T(n+1) troop directly always costs less total resources than building T(n) then upgrading it to T(n+1), though the upgrade step alone costs less than building new. Strategic guidance: avoid mass-building troops below T9 (wastes resources/speedups); aim to start serious training at T10 (Keep 25) or ideally T12 (Keep 30); once at Keep 33+, stop trying to fully ghost troops and instead build large defensive numbers, targeting roughly T13: 500k->1M, T12: 1M->1.5M, T11-and-below: 100k->500k per type over time (~2M+ total troops as a baseline to withstand Keep 35 attacks, buff/debuff-dependent). Upgrading Mounted troops preferentially to T12/T13 is recommended early for monster hunting (e.g. needed to beat Witch 3/4, Warlord 3/4) since Mounted is less useful for city defense.
Original research source: Evony Guide Wiki