The Watchtower provides detailed scouting reports and warns you when troops march toward your city; ALL scouting features are fully unlocked at Lv41. Scout feature-unlock levels: Lv4 target's F/L/S/O; Lv8 number of incoming troops; Lv11 number of target's subordinate cities; Lv14 number of defending troops; Lv17 Walls HP, reinforcement amount, Archer Tower level, full trap list+levels; Lv19 Truce Agreement Informant; Lv21 type of incoming troops; Lv25 type of defending troops; Lv27 type of reinforcements; Lv30 target troop buff; Lv37 Main City Defense Generals; Lv41 reinforcements buffs. It is tied to the Forge: the Forge cannot reach Lv26+ unless the Watchtower is also leveled (Forge Lv27/30/33 dragon/Ares/Achaemenidae equipment depend on it). Buildable at Keep 6 (also needs Farm 1); Keep+Farm gated. Cost F/L/S/O, speed to +610%, Power ~62M at Lv50. For a bot, Watchtower level determines scout-report richness and gates late Forge upgrades; mostly a PvP-intel building rather than economy.
Original research source: Evony Guide Wiki