The goblins of Cinder are not the filthy little bastards of mainline D&D. Well, they are filthy little bastards, but they are also much more. The goblins are a fey people, related to elves and fairies and dopplegangers. (Incidentally, Cinder fairies are basically elves with butterfly wings.) They are not kin to hobgoblins or bugbears. It was an influential but completely clueless human sage who decided that obviously green sub-elves, Andorians, and pumpkinheads were all branches of the same family tree. That same dude also insisted in his bestselling bestiaries that aardvarks were a subspecies of dragon.
As a fey race the goblins are a magical people, but they don't cast spells like elves or change shape like dopplegangers. They have a body of magical lore but very few members of the race know more than one or two tiny bits of magic. Goblish moonbeam farmers probably don't know any other magic but that necessary for lunar agriculture. Goblin armorers probably only know how to make rag mail armor (which protects as plate+1 as long as it goes unwashed) and little other magic. One of the more common bits of Goblin magic is the ability to summon and use Goblin Doors.
Most Goblin Doors look like smaller versions of standard dungeon type doors. Occasionally spielbergian light will shimmer and shine through the cracks in the planking or from the keyhole. Other fey races can open and close Goblin Doors, but the summoning and dismissing of them is an unknown art to even them. Goblin Doors usually lead to other rooms in various parts of the same dungeon, but 1 in 6 lead to more distant locales. Note that when open the space on the other side of the door might be plainly visible, it might be hazily perceived, or the doorway might be full of sparkly lights making surveying the space beyond impossible. It is believed that once summoned, the same Goblin Door will always lead to the same destination.
Reports of Goblin Doors leading to places that have no Goblin Door to take one back are nothing more than baseless rumor. Honest.
5e Backgrounds for The City
29 minutes ago