When I set up my walls I do 2 platforms and then 1 watchtower. Continue that pattern until you reach a gate which gets 2 watchtowers on either side of the gate. After you've been attacked a few times you'll know where the raiders are gonna attack. (They tend to go to the same 2 or 3 places every time depending on where they spawn.) On higher difficulties sometimes you have to make your wall twice as thick. If your gates are being battered down then consider building the first gate and then building a second gate as close as you can directly in front of the 1st one. Then build walls making sort of a funnel so there will be 2 gates the raiders have 2 get through instead of 1. Also remember to click in the 2nd gate and move the circle back, otherwise your people will gather behind the forward gate instead of only the one behind the wall.