The best way to place barricades are in the diamond position due to the player still being able to travel through but zombies can not.
This pattern looks like so: (Apologies for the dots, the formatting messed up when just using spaces)
...............X
...........X......X
.......X..............X
....X....................X
.......X..............X
............X......X
................X
Buy the barricades while on sale and fill the inside of your little base with turrets that are mounted with a shotgun. Just keep your ammo up and keep killing.