The Combat Engineer is responsible for the Team's field fortifications and the well being of placed structures. He can set up autonomous operating turrets and barricades that attract zombies.
Sentries + Cades
This build gives you everything you want from an engineer, at the right time. The first level in engineering helps you keep the bunkers during the 2nd mission alive, after that we rush for turrets. Once these are set up we get barricades and after that the Energy Aura with SS2.
Alternatively rush Security with LO si for better base DPS.
Avoid planting barricades prior to Thresher Maw spawn, as it has a tendency to directly bring them into the base.
1x Engineering, 4x Security, 3x Engineering, 4x Soldier Skills
PU, IN, EN - for you energy hunger
LO - MGL and Railgun turrets are pretty strong, and you get a third turret
ISS7,AA-12 after the Enforcer have theirs
KI - if the medic and fss have one/don't need it