Star Wars: The Force Awakens Comic Con Trailer Breakdown – 46 Things You Need To See
12. C3PO's Missing Arm
Further confirmation of C-3PO's missing arm - and indeed the red colouring confirming that he's scavenged the new limb from a TC-3PO after losing his more shiny appendage.
The background seems to suggest that Threepio has returned to the Rebel base on Yavin as featured in A New Hope. That seems to suggest that the rumours that the Green Planet mentioned thus far in promotional material is in fact Yavin 4, which will be the home of both Leia and Maz Kanata and the pirates.
The fact that the pirates live in a castle fits with the idea that the sequence showing the First Order assault on the ruins is their attempt to wipe out the pirates.