Scale Citadel is an ancient fortress temple found in Old City, built to house and protect the Furox Bone-Mark from the outside world. It resembles a large-horned dragon head from the outside, with a glowing mouth and the entrance located at the dragon's throat. The entire fortress is riddled with traps to help prevent anyone from reaching the bone-mark.
Upon first entering, there are thin but tall pillars that line the halls. These pillars provide the central lighting source for the temple, running on Red Draconium. There are lights at the top of these pillars shaped like the symbol of the Dragon Flares Crew (which was originally the symbol for the Red Draconium Empire whom the Dragon Flares are currently descended from). These lights are very large and shown to be larger than the teenagers themselves. At the base of the pillars lies another light source of gentle-glowing Red Draconium from within cage-like structures, portraying the appearance that each pillar holds a fireplace at its base.
The first passageway reveals the first challenge and set of traps; extremely large guillotine-axes which drop from the roof to the ground and slide out of the walls left to right. These axes swing away, and the rider and their dragon must be fast enough to race through when the saws have retracted and provide a clearing to pass. The second passageway leads to a hallway filled with stone ledges that stick out of the wall from either side of the hallway. These are motion-activated and when triggered, the ledges from both sides come together in the center of the hallway, interlocking with one another and crushing anything that stood in between them. They appear at various heights too, so the rider and dragon cannot simply jump over each ledge, and must instead manoeuvre accordingly to get past the ledges before they have a chance to lock-up.
The final room rests on a narrow winding ledge over a large precipice. The walls next to them are lined with large dragon heads and ahead is a smaller cave, who's entrance is decorated by a stone dragon head, where the bone-mark rests. Along this path lies a Draconium trigger-pad, shaped again like the Dragon Flares Crew symbol, which activates when a dragon or person steps on it. Once activated, the stone dragon ahead beings to close its mouth and seal off the smaller final room, whilst the dragon heads on the wall shoot projectiles along the path which resemble the projectiles fired by Green Ramming Gear. The rider and their dragon must be fast and agile enough to dodge or defend from the projectiles as well as make it into the smaller cave before the dragon mouth shuts. The intended mechanism however, is designed to keep said rider and dragon trapped within that smaller cave forever. The end of this smaller cave, where the bone-mark is located, has many carvings along its back wall, most likely containing the ancient stories of the original Furox.
This citadel makes its one and only appearance in Episode 5: Fanning the Flames. While the mechanism was supposed to keep Kitt Wonn trapped inside forever, she wedged a rock in between one of the mechanism's cogs and temporarily shut down the whole citadel's defence mechanisms.