Thessia is falling under the concentrated assault of the Reapers, but there's still plenty of fighting to be done. The sectors of Firebase Goddess, although smaller than those of Jade, are better designed for defensive play. Several of the interior areas encourage close-quarter fighting, a dangerous position during later waves.
Exterior
The Exterior is one of the larger areas of the map, and is the starting location of the player squad. The Balcony overlooks the landing zone, so be aware of this when holding down the fort for extraction.
Balcony
A small sector overlooking the landing zone, this area is tight and offers little in the way of cover. It's best not to stay too long or become too attached to this area.
Elevator Access
This is a box shaped room with a currently non-functioning elevator. The Great Hall and the main Exterior areas are accessible from here.
Entry Hallway
Leading directly to the Great Hall, this sector is flanked by the Lab and the Main Corridor, and it even connects to the Exterior. In short: not a safe place to try and hold out in during a big firefight.
Great Hall
The Great Hall is the largest interior section of the map, and it can be accessed from nearly every other sector. Fortunately, this area is actually quite defensible, with lots of cover to protect you from potential snipers, and long lines of sight that allow you to engage enemies from afar.
Lab
The Lab is another small area located near the Balcony and Exterior, and has a passage that leads into the Great Hall. This tight location is another 'corner' that can seal the fate of careless players who find themselves stuck here during heavier waves of enemies. Unless you have pressing business here, like an objective location, it's best to keep moving.
Main Corridor
The Main Corridor is located next to the Lab and has direct access to the Balcony and the Great Hall. It's best to consider this and the Lab as one combined section, and to transition between them as the situation dictates.