Near everyone objective and/or command post. Or f.e. on the way of the maintenace bot (container city preview)
It'll get you a lot of kills and you actually help the team.

However, that's a theoretical, If I'm playing engineer, I'll put them as close to the frontline as possible, still defending objectives. I won't just put them at the final objective at the beginning.

I can also see mines in small alleyways, but it depends on what kind of mine it is. (If there even are different kinds)

Are sentries solid? Just wondering if it could be used to reach even higher areas.

Any updates on turret placement yet? So far, all that I know about it is that you have to take the time to set it up, and that you can't place it on terrain that's too uneven. Hopefully the restrictions won't be too severe. I'm looking forward to creeping behind enemy lines, and placing turrets in places that are... inconvenient to the enemy. Imagine that: you've just respawned; you're dashing to the frontline to re-join the fight. As you run by a building, a turret just to the side of you, up against the side of the building and out of sight for any approaching reinforcements, locks onto you and empties a clip into your back. Dead. Before you even get back into the fight. And to make matters worse, who's to say how many of your teammates will fall into the same trap? How long will your team take destroying it? How much ground will the enemy gain, while you're away from the front lines, trying to destroy this turret? What about the engineer - is he still skulking around, hunting anyone who may have bypassed the turret? Or is he supporting it, taking out anyone who evades its tracking, and repairing it when it becomes damaged?

I don't think it'll work that way. I don't think turrets shoul be placeable near spawns anyway. *Besides: Dead before you even enter the fight isn't that nearly the same as spawnkilling*
If you can sneak behind enemy lines and reach such a spot, either your team is great in pushing or the other team is bad at defending. And there's also the map layout. There should be plenty of places to hide or ways to get to the turret from behind. If you can't get the turret from behind, then it will probably be placed in an heavily defended area or in a small allyway where almost no one comes.

I think turrets will just help pushing forward or defending objectives. Not to get crazy amount of kills on unreachable spots near spawns.

Yeah, this is pretty much it. An engineer gets automated fire support, and can lay mines. To me, at least, he seems perfect for disruption tactics. Everyone will be entering the game wary of an enemy operative, disguised as a teammate, creeping up behind them for a backstab (and mines too, heh, but whatever)... but what about a brazen, in your face ambush by an engineer and his turret?

As for spawn camping, I didn't mean that at all. I'm with SD on that one - it's cheap, and ruins the experience for everyone. This ambush that I had in mind would take place somewhere well between the enemy spawn point and the front line.

speaking of engineer tactics: where do you put your mines? in a high traffic area or protecting your teams flanks?

that's a great idea actually