Forts/Fortified Lines

[Orel]

What is the effect produced on troops when they are inside fortified lines?

[PhilThib]

Mostly the same as forts, except a slightly lower protection modifier

[Orel]

Mostly the same as forts, except a slightly lower protection modifier

Would you be able to give a bit more information?

I would like to learn what forts or fortified lines provide to the troops positioned inside.

[Baris]

Units garrisoned in fortified lines gain 800 entrenchment. It is via option Verdun. I don't know if other units other than native garrison also receive the bonus when inside. But it is easy to check in game or battle log.
Forts maybe have max 500 entrench value. But regarding frontage in assault I think assaulter have the same frontage and negative troop quality check against forts&fortified lines. You can read all those in battle logs.