The War Hall increases rally capacity for rallies you host: higher level raises the max participants you can accept when you start (host) your own rally. Buildable at Keep 9. Leveling requires first raising the Embassy -- each level from Lv2 requires the matching Embassy level (Embassy 2 at Lv2 ... 50 at Lv50), and higher levels also the matching Keep level. No Duty Officer section and no troop-tier Unlock column. All menus stay usable during upgrades. It also provides the Alliance War screen (monitor rallies/scouts by alliance members or enemies -- same as Alliance > Alliance War). Costs: Lv1 1,000 food/1,000 lumber; Ore at Lv12; Lv25 ~11M/14M/15M/14M; Lv50 ~52.5B/66.6B/68.6B/66.6B. Power 10 to 72,000,000. Speed +0% to Lv17, +30% at Lv18 to +610%. The Workshop requires matching War Hall levels for Lv36-50 upgrades, so War Hall level is a prerequisite for high-level siege progression. For a bot: the host-rally-capacity building, gated behind Embassy (and Keep 9 to start), distinct from the Rally Spot (your own solo march size).
Original research source: Evony Guide Wiki