XCOM: Enemy Within trailer introduces the Exalt, hints at base defence missions
Here's a trailer that focuses on the Exalt: those covert enemy operatives that Tyler revealed in his XCOM: Enemy Within preview. They're an enemy within, you see. Within the expansion, within your species, and, as your trailer shows, within your own base. While the trailer consists entirely of cutscene, it does suggest that base defence missions could be making a return.
If that is the case, then hurrah, base defence! The original X-Com's alien infiltrations were an excellent way to escalate the danger of the mounting invasion. It's not quite clear how that would work given the new game's multi-level dioramas, but hopefully the engine can cope. I found XCOM: Enemy Unknown frequently buggy when it came to mouse selection during interior battles over multiple floors.
One of the confirmed Exalt interactions involves you sending counter-operatives to infiltrate their own hideouts. For more on this, head over to Tyler's hands-on report, where he takes you through the tactical choices at the heart of the conflict against these enemy cells.