The Warehouse protects resources: it increases protection from enemy looting, protection of food from soldier upkeep (a larger amount held without being auto-consumed as upkeep), and the storage limit for gathered resources and relics. All menus stay usable during upgrade. It is Keep-gated 1:1 (Keep n per Warehouse n) and is a prerequisite for the Pasture (raise Warehouse first). Only milestone: Duty Officer at Lv23 (increases protection from looting and upkeep; ideal Philippa, then Guinevere). Five ways to increase food NOT consumed as upkeep: (1) raise Warehouse level; (2) research Advancement > Adv Silo (max +200%); (3) defense general Shajar al-Durr (+200%); (4) Warehouse duty officer (Junior +20%, Medium +40%, Senior +60%, Special +100%, Super +150%); (5) Lv30 Monarch Talent Troop Upkeep (+100%). These stack multiplicatively on base capacity: Warehouse Lv30 base 25M -> 160M with Shajar 200% + Medium officer 40% + Adv Silo 200% + Talent 100% (540% total); Lv35 base 70M -> 525M with a Super officer and 600% total. For a bot, Warehouse level plus these buffs define the safe food buffer that survives upkeep drain and raids -- critical for large troop counts where upkeep would otherwise consume city food.
Original research source: Evony Guide Wiki