The Workshop is the siege-machine training building; higher level unlocks higher siege tiers plus training capacity/speed. Builds Lv1-50; early levels require Keep 6 plus a matching Sawmill level (Sawmill progression gates early Workshop). From Lv26 requires Quarry (and Trap Factory at Lv27/30/32/35); from Lv36 requires the matching War Hall level, plus Stables at Lv44/49; higher levels also need matching Keep. Siege tier unlocks: T2@6, T3@9, T4@11, T5@14, T6@16, T7@19, T8@21, T9@23, T10@25, T11@27, T12@30, T13@32, T14@35, T15@40, T16@45, T17@50. Blazon at Lv8. Key milestone: Duty Officer at Lv29 (increases Siege Training Speed and Capacity; ideal Gwon Ryul, Mikhail Kutuzov, Roger II). Costs match the others (Lv1 1,000/1,000; Lv25 ~11M/14M/15M/14M; Lv50 ~119B/152B/156B/152B). Speed +0% to Lv17, +30% at Lv18 to +610% at Lv50. For a bot: siege units have NO upkeep and carry the most load, making them ideal gatherers -- so the Workshop is doubly valuable (siege combat tier and gathering fleet). Note early levels are Sawmill-gated and later War-Hall-gated.
Original research source: Evony Guide Wiki