The Trap Factory manufactures defensive traps that fight in wall defense, and repairs damaged siege machines (siege are repaired here, not at the Hospital). Four trap families each counter a troop type: Trap (anti-Ground), Rock (anti-Ranged), Abatis (anti-Mounted), Fire Arrow (anti-Siege), all 1,500 range. Higher building level unlocks higher trap tiers I-X (Trap I at Lv1, Trap V at Lv22, Trap X at Lv46; Fire Arrow I at Lv4 to X at Lv50). Each trap costs resources and time and adds Power. Buildable at Keep 8 (also needs Sawmill 1); early levels gated by Keep and Sawmill. Milestone: Lv19 Duty Officer (increases Trap Building Speed and Damaged Siege Machine Capacity; ideal Tachibana Dosetsu, Otto the Great, Askia Muhammad I). Building traps Lv6+ (needs Trap Factory Lv26+) is also how you farm Adv Troop March Speedup in the Gather Troops event. Cost F/L/S/O, speed to +610%, Power ~62M at Lv50. For base_dev, Trap Factory level governs both trap tier/strength and siege-repair capacity.
Original research source: Evony Guide Wiki