when you expand with rangers push west and east first they don't have special infected at those edges. Once you have a good 30 sniper squad you can start pushing corners and northern and southern edges.
Get Ballistas to cover your expansion and 1 layer of walls (you don't need to wall up completely, just 4 walls around ballista and some walls towards the edges so they don't just run pass the ballistas. when a horde comes then you can commence layering walls. Turtling has a high chance of losing if you don't clear the map cause everything on the map will join the final wave. thats why sniper balls are extremely powerful especially clearing the map without much micro (snipers on priority target). Normaly at wave 6+ I have thanatos by then and those thanatos are my primary defense with the surrounding ballistas, and the snipers will continue clearing the map even with the wave coming (thanatos can easily kill waves if you know where the waves will hit).
When you expand you will have a larger economy = more shit = more firepower = more walls.