Installing a Town Watch is fairly painless, and can be done from the get-go (but at your Town Hall, so this is perhaps not the most advisable idea early on). The moment some invisible people agree to keep watch over key points, your buildings have the LOS increased by 4. And since the invisible people figured out that one can probably see farther from a higher position and it'd also be nice to cause some damage to the attacking army before your troops get there, you can also build Watch Towers at that point. The Town Watch enables your men to install a Town Patrol later on.
Available to: All
Researched at: Town Hall
Cost: 75 Food
Effect: Buildings +4 LOS, Watch Tower enabled. Unlocks Town Patrol.