An archer's tower is a type of fortified unit consisting of a tower built atop the city walls. It provides a vantage point for archers to shoot enemies. The height (level)of the wall determines the shooting range and thus the effectiveness. The actual number of archer units in the city has no effect on the performance of archer's towers.
Construction time is affected by Construction Research and your Hero's Politics Skill. Construction Time = BaseTime * (0.9 ^ Construction level) * (0.995 ^ Politics level)
Archer Tower Range = (((WallLv+Archery)*0.05)+1)*1300
Like all Evony defences, it is only effective in huge numbers.
Most players consider Archer Towers (ATs) the most important defense on your wall.
Edit : For those unfamiliar with computer arithmetic signs, the author's "^" in the formulae means "raised to the power of...", so (0.9 ^ 3) means (0.9 x 0.9 x 0.9) (I was confused beforehand, thinking it was a Multiply sign, with no spacing in the terms)
75-85% of all your total defenses should be archer's towers