Watchtower what does it do?Edit
The Watchtower tells you two things.
First, it tells you if you're about to be attacked by popping on your screen a red circle with an exclamation point in it. Beside that, is the time till it hits.
It also shows where other players armies are going, if they are in the circle. The white circle on your map is how far your tower can detect enemies.
It may be that you don't detect an opponent's scouting if your watchtower is lower level than your opponent's.
Well, where is it?Edit
If you go to the world map and click on your city three options will pop up. The one on the right, a tower, is your watchtower, you can upgrade the tower here.