Aegis Defence Line
An Aegis Defence Line is an Imperial barricade built from crenellated armoured sections that link together into a solid shield wall. Their simple design means that they can be built and deployed at great speed, lending them the nickname of "instant fortresses."
Aegis Defence Lines are ideally suited for commanders wishing to hold ground in enemy territory or establish defensive perimeters until such time when larger, more permanent fortifications can be constructed.
Largely unchanged from 7th, with the notable exception of how cover works in 8th. It's far better for moderately-armoured static infantry (no longer giving any bonus to non-infantry) like devastators. Cowardly Tau or Guard can no longer cheese out 2++ from it, but that's not really a dreadful loss. The line cannot be attacked or damaged, although if you buy it gun upgrades those can be.