You set up 5 turrets and a minefield.
Our Light Operative parkours his way over and behind the mines (because he can see them even when his teammates can't), and hacks your turrets, then shoots the mines. to clear a path for his teammates, captures the CP, and Firewalls it. So even if you find a way past the turrets that used to be yours, the CP takes longer for you to hack than normal, giving our forces time to reinforce the position.
Also, we don't know what (if any) limitations there will be on setting turrets - it's likely you won't be able to have more than one out at a time per Engineer. So to set those 5 turrets, you probably need 5 Engineers.
It's been implied there's a maximum limit to the number of mines a single Engineer can set (because there's an ability which increase the number listed on the Brink Wiki)