Overview
 

The Engineer is probably the most strategy intensive classes in the game. He is great in defense and can be very useful in an offensive role as well. He has 100 max armor (jacket only) and carries a blaster, shotgun, super shotgun, Armor Piercing Dart Gun (APDG), machine gun, Tesla Coil, Magnatron grenades, normal grenades, and Shock grenades.

Defense

Sentry placement is the key to a good defense as well as keeping your teammates aware of whats going on. Putting the sentry gun where someone can plink it with grenades or just chuck a few rockets at it leaves to very alone and vulnerable. Grapple somewhere and put it just above a door, or  just anywhere above ground. Also, dont expect the sentry to do all the work. Powerful though it is its not too difficult to kill and being immobile cannot dodge any munitions. The super shotgun is probably the best one on one weapon the Engineer has. Wait around a corner and blast anyone who comes around, and they'll either take heavy damage or go after you while the sentry rips them apart. Alarms are extremely useful too. Set them in a dark corner where they wont be seen and you have a very effective early warning device, allowing you and your teammates to prepare. On defense one should spy-check every single person who walks into your flagroom. If they are a spy they'll probably either run for your flag or run scared because they've been found out. If they run for the flag they're pretty much dead because a spy is quite weak and cant take you and a sentry gun blasting away at him. The always annoying Marines or Cyborgs who like to hide around a corner and chuck rockets and grenades into your flagroom can be removed with one of the Engineers two prized weapons. One is the loathed Magnatron grenade which sucks opponents towards it before it explodes, dragging people from behind corners and possibly into the path of your sentry gun or Tesla Coil. The other grenade is the Shock grenade which, whcn it explodes, gives everyone in the area a healthy dose of Tesla Coil. Since the new Spy feign death skill has been introduced one should always shoot corpses that lay around and chunk them, any one of them could be watching you......
 

Offense

While most people dont see the Engineer as being offensive he actually can play a powerful part in offense. He can set up a Sentry Gun and Supply Depot in a foward position in the enemy base, thus re-arming and defending his fellow soldiers and if he is backed up with a Nurse, they can be healed as well. The Supply Depot can be used to play a nasty trick on the opposing team. If you make it into their flagroom with an attacking party you should set up a depot in a corner of their flagroom and return to your base. They will probably try and use it, so when it messages you... BOOM! Also on offense the Tesla Coil is actually quite good at killing Sentries, providing they dont get you first. The Armor Piercing Dart Gun will give your teammates 5 armor so you can re-armor them on the run.