In many ways, the engineer is the most important class in Enemy Territory. They are, of course, the only class that can build structures, repair vehicles, and dynamite objectives. In fact, Rail Gun is the only map that can be won without the aid of an engineer - on every other map, each team needs at least one to achieve victory.
The first decision to be made as an engineer is which weapon to choose. The Thompson/MP40 is a good all-around choice, with a good balance between accuracy, power, and firing rate. You really can't go wrong with a good submachinegun, after all. The rifle is the only other choice for an engineer - K43 for the Axis, and the M1 Garand for the Allies. Each shot packs more than double the punch of the SMGs, but the clip is limited and you're stuck with semi-automatic fire. The rifle is also extremely inaccurate when you're running around and jumping, making it a close-range weapon unless you have time to crouch or (ideally) go prone. For the middle ranges, the pistol is probably a better choice.
However, if you want access to the rifle grenades you will, of course, need to choose a rifle. The rifle grenades are nearly equal to the Panzerfaust in destructive capability in the hands of a practiced user, and the ability to lob grenades over 100 yards with pinpoint accuracy can consistantly result in one-hit kills. Level 1 engineering gives access to 8 of them, which should keep you in explosives for a long time. Just remember that the grenades are fast, but not jet-propelled; they will drop due to gravity, so aim 5 to 20 degrees above your target as well as leading them. Each grenade also drains half of your power bar, so keep a close eye on it when charging a demolition point. Not only is it possible to drain all of your power and be forced to rely on the rifle itself, but that's the same power bar you use to plant dynamite or lay land mines. Wouldn't want to have to camp a heavily-guarded objective by yourself for 20 seconds with a pistol before you can even stick a bundle of explosive love on it, would you?
So rifle vs SMG, which is better? For the Axis it's a pretty clear-cut decision - the K43 has a 10-shot clip, the rifle grenades are too good to pass up, you're not going to be planting dynamite very often, and the ten bullets you get is enough to take out any attacker at close range. For the Allies it's a harder question. The rifle grenades ARE excellent for clearing out clusters of Axis defenders, but the rifle itself is decidedly subpar. You get the same poor accuracy as the K43, but each clip has only 8 bullets and must be completely emptied before you can reload. That means most battles will drain a full clip, because you can't afford to start a battle with just two bullets in the gun. If you expect to engage mostly in long-range combat go for the rifle, otherwise you might be better off with the Thompson.
All weapons aside, how the engineer should be played primarily depends on which side of the assault they're on. The Axis find themselves defending on most maps - when there are objectives for the Allies to demolish, it's vital that at least one engineer remains within at most a 15 second run of the objective. Nothing will annoy the Allies more than the sound of "dynamite defused," and nothing will annoy your team more than losing a match within the first 10 minutes. In the early moments of the map you're free to roam and build stationary fortifications, so take advantage of it. Not only will you gain quite a bit of experience, but you'll have them standing before they're needed rather than trying to build a barrier (for instance) while under fire from tanks and airstrikes.
In "multi-goal" maps such as Wьrtzburg Radar that require either building or demolitions before the Allies can achieve their primary objective, however, a quick engineer can usually slip in to defuse dynamite, destroy a bridge with a quick plant, and so forth. The longer you can delay the Allies from getting to their real goal, the less time you'll need to defend it. So on maps like Siwa Oasis and Radar, for instance, guarding the destroyable walls can buy your team at least a minute per unsuccessful attempt. If you can keep a secondary entrance closed, you'll also make it that much harder to get past your team's defenses. Remember, an engineer taking time to plant is a nice, juicy target.
And finally, use the land mines! The engineer's other specialty, each team can have up to 10 land mines placed at any time. Each depletes half of your power bar to drop, and they must then be manually armed with the pliers like dynamite - in other words, setting up a comprehensive landmine defense takes some time. They're best placed at the choke points or near goals as a last-ditch tactic against opponents who have gunned down (or bypassed) all of your defenders. Keep a careful eye on the death messages if you do this, and run back to check and re-plant mines. Don't use the exact same places, though... shuffle them around a few feet each time to keep defenders guessing.
On the Allied side, you're more than likely going to find yourself on the attack the majority of the time. When you can, stick with your team and try to invade en masse. With a good medic at your side you're far more likely to survive and accomplish your goals, and that's what the game is all about. Stay out of the first wave, shooting rifle grenades past your teammates to hit likely ambush points and camping spots. When the coast is clear, make a run for the objective and do the deed!
Another viable tactic is the "commando run." Head for the back entrance (most maps have one) while your team distracts the enemy. Avoid conflict and head to the goal by the safest route possible. With a little luck the defenders will all be on the front lines, and by approaching from an unusual entrance you may be able to get the drop on any stragglers. Avoid using your power bar on the approach if possible; your first plant should draw the attention of the entire Axis team if they're paying attention, but you should have time to place a second stick before the defenders arrive. If you can buy even a few more seconds defending the goal, a single engineer should run out of time trying to defuse a pair of dynamites. I've seen this done successfully once with a pair of engineers.... the sight of a lone Axis engineer trying to defuse 5 bundles of explosives made me laugh until my sides hurt.
One of the biggest problems with the lone engineer is landmines; if you're significantly ahead of your allies, odds are no Covert Ops will have scouted the area. On the other hand, engineers have the least to fear from landmines as long as you remember a few simple tips. First, the mines are triggered when stepped on, but only explode when you step off of them. If you aren't actively engaged in a firefight, stop as soon as you hear the distinctive "click - hiss..." Pull out your pliers and get to work. You'll defuse the landmine, gain some XP, and be on your way. If you can achieve rank 4 engineering you'll have even less to fear, because you'll be issued a flak jacket that protects against half the damage from all explosions.
Here's the lowdown on the Engineering skill, taken right out of the manual. The manual was installed with the game, so check the Wolfenstein: Enemy Territory group in the start menu to read the whole thing.
This is an Engineer-specific skill that rewards players for constructing/demolishing objectives and the use of explosives. Engineers earn 3 XP for repairing a vehicle or MG. They also earn XP for constructing or destroying constructible objectives: 5XP for a 50% of Power Bar objective such as repairing an MG nest, 7.5 for a 100% Power Bar objective like repairing a tank and up to 10XP for destroying an Objective. Engineers also earn 3XP for a Rifle Grenade kill, 4XP for a Land Mine or Dynamite kill, 4XP for defusing an enemy Land Mine and 6 points for defusing enemy Dynamite.
Engineering Level 1: Improved use of Explosive Ammunition
Inventory includes four extra Rifle Grenade rounds and four extra Hand Grenades.
Engineering Level 2: Improved Dexterity
The experience of handling explosives in combat allows Engineers to arm and defuse Land Mines and Dynamite in half the time.
Engineering Level 3: Improved Construction and Destruction
The expertise of the veteran Engineer means that constructing and repairing objects and setting Dynamite or Land Mines uses 1/3rd less Power Bar charge than normal.
Engineering Level 4: Issued Flak Jacket
Only the most expert Engineers who have proven their ability to survive on the battlefield are issued with these expensive Flak Jackets which provides the player with 50% damage deflection from explosive weapons.