Wat is a defence line?

Wat is a defence line? I can't find any information about it.

A defence line (or d-line/Dline for short) is a defensive way of protecting your citys, or used as an offensive way simmilar to the castles in wales used in the middle ages. You make them with an assortment of units, but it works best with miltia and infantry, as their mainly defensive units. You simply place one where you want the line to start, then pick up another but move it in the proximity of the other so a line appears inbetween them, once it does repeat the process until your wall/fort is complete, cheers!

Another advantage of using militias and infantries on a defense line is that they get a +1 bonus to defense while on it.

Also, you can't build defensive lines with any stealth unit or with planes.

Actually, you can build defence lines with un-stealthed marines