The Guard Tower is the upgrade of the Watch Tower, a defensive structure with massive line of sight. The Guard Tower is more durable and fires a heavier arrow with increased bonuses, but does not fire any farther. Guard Towers upgrade further to Keeps.
The Bretons and Flemish, however, do not upgrade this tower. It seems their defensive technology just hasn't caught up.
Century: 14th (Requires previous upgrade for 250 Food, 100 Wood)
Cost: 125 Stone, 25 Wood
Armor/Pierce Armor: 2/8
Range: 8 (MR 1)
Special: +9 vs. ships and lancers, +2 vs. spearmen.