My TH level is 5 now. Which level do walls should be?

Usually around 3-5 would be best

walls must me maxed out to slow down the intruders.

In town hall 5, your walls should be at level 5 as well.
Level 5 walls is the maximum wall that you can have for this level of town hall.
The color of this wall is gold.
It costs 30,000 gold each wall when upgrading it from a level 4 wall.
Level 5 walls has a much greater hit points which is 1,400 compared to a level 4 wall which only has 900 hit points.
Upgrading walls does not require a long time to upgrade unlike other defensive buildings you have.
Maximizing your walls helps you defend your village by slowing down enemy troops from when attacking.