In RA2 they had Terror Drones, yeah, spider thingies that kill tanks. But that's definitely not what I meant by "mobile crawler."
THIS is a Crawler:
http://cnc.wikia.com/wiki/Crawler
There are two of them, the GDI the prominent unit on screen, the red/black Nod one in the background. The two smaller (!?) massive walkers are Mastodons: the latest iteration of Tiberian Sun's Mammoth MkII walker.
The Crawler takes the place of the player's base in TT. In fact, unless you're playing the defensive class, you can't build structures at all. Crawlers build your units, and can store units inside them for protection or strategic deployment. They seem pretty heavily defended, and have different upgrades that can be earned.All crawlers can deploy into a construction yard, like classic MCVs. And depending on whether they're deployed or not, their capabilities will change.
Additionally the Crawler is slightly different for each class. The legged crawlers above are from the Offensive class. Support crawlers fly, while defensive crawlers use wheels.
That brings us to another new feature of TT:Respawns. C&C4 games are treated more like FPS matches than RTS matches. If you "die" you can respawn, and when you respawn, you choose your class. You can even self-destruct your crawler if you want to force a respawn to pick a different class. This holds true for both multiplayer and singleplayer. How that's going to work with victory conditions, I am not sure.