What stategy should I use when playing as the engineers?
The risk of getting boxed in is always something you have to pay attention to. (Of course, the risk is greatest for this with the Chaos Magicians)

I only played the engineers once (and won with about 135 points), so I don't have much experience with them.  The stronghold ability is very strong but should be used situationally. What I mean by this is that you should not focus too hard on building bridges. I had my first point scoring bridge in round 2 (because for scoring points there have to be 2 buildings at either end of the bridge).

Very important (like always) is the position of the starting dwellings. I chose C11 (lots of possible bridges) and E7 (mainly for power leeching), but of course this depends from game to game.

I also built only 1 bridge with 2 workers, the other 2 bridges I built with the power bridge action. I think you should always do it like that if it's possible.