Note
When you build 1 of something, you only build a part of a building; each part takes 1 land. Having 1 part of a 10-part building is less than half as effective as 2 parts of a 10-part building. The closer to the completion of a building, the more efficient it becomes. For example, 10 of 10 Training Fields is 100% effective while 9 of 10 Training Fields is only 81% effective.
Back to Buildings
Guard Towers work much like Outposts and they help you to prevent enemy covert operations. They require 2 parts to build a complete guard tower and it boosts the defence of 500 troops (increases their attack and absorbtion by 40%) and the covert defence of 100 Rangers on top of that. The Weapons, Armor and War Machines also receive the bonus.
Gaurd Towers increase troops by a factor of 1.65x current combat power. The Rangers sitting in Guard Towers also get double defense against assassination and looting. Also note that if you have 1 mage per troop, you will need half as many outposts as these troops get the bonus their counterpart troops use.