Should the speed to get to the next level be slower than it is if you dont have engineers present?
Also could the use of the card Defensive Works would need an structure, depot or a port to use it?
Then you could bring it back up to level 5-8 from the start?
I think that field fortifications need engineers to build fast and that should be the only way to do it in the field.
The card would represent well the possibility to buy civilian / slave labour to fortify locations behind the frontline.