The Embassy shows reinforcements to you/allies and lets you send back or recall them. Raising its level gives: (1) Reinforcement -- higher capacity to receive reinforcing troops; (2) Construction -- higher number of alliance help received and greater help-time shortening; (3) Transporting -- larger transport amount, more transport times, larger received amount, reduced transport-fee rate. So Embassy level directly governs alliance-help count -- valuable for a bot that requests help on every build/research. Buildable at Keep 9 (also needs Market 1); Keep+Market gated (raise Market first). Raising the War Hall requires the Embassy first. All menus stay usable during upgrade. Main milestone: Lv27 Duty Officer (buffs Reinforcing Troop Attack/Defense/HP; ideal Queen Anne or Heraclius). The page gives no numeric help-count values -- only that higher Embassy level raises the number of alliance helps. For base_dev priority: the Embassy is high value because it raises free-help count, which is free time before any speedup; keep it near Keep level.
Original research source: Evony Guide Wiki