Strongholds tasks guilds with clearing the lands around the keep of monsters before building structures and tower defenses as well as siege components to attack enemy guilds. “Strongholds gives our Neverwinter players the ability to create their own stories of banding together with their guild to forge the tales of their rescuing of lands overrun with orcs and defending their turf from invading players,” says executive producer Rob Overmeyer.
You’ll find a trailer for the expansion just past the break.
Source: Cryptic press release