Tubet is an Elbe castle located in the Tuba Mountains, Special Region, near the border between the Empire and Elbe. The castle appears to to be built at the opening of a major mountain pass, with a large gate and numerous towers defending the location.
The structure of the castle itself is similar to a western European castle, with tall stone walls and towers topped with crenelated battlements intended to be manned by archers and other defenders during times of siege. The main gate, as well as the two sub-keeps, would pose a formidable threat to any besieging army in the Special Region, though, like all fortifications constructed by Special Region natives, these would obviously be completely useless against an attacker with the technology level of the JSDF.
The fortress of Tubet consists of a large wall covering the entirety of a narrow gap between two vertical cliffs, centered around the main gate, which appears to be a sturdy wooden structure, presumably reinforced against ramming. Within the wall are six towers for stationing archers and other defenders. The wall reaches the top of the cliffs, and a walkway continues from the wall along the top of the cliffs up to the two keeps.
Rather than a single central keep typical of European, and presumably Special Region castles, Tubet consists of at least two, possibly three separate keeps, each of which could be considered a separate castle in its own right. The main keep is to visible to the largest of the three, and located to the right of the gate in the image above. The main keep consists of a the central round keep itself, as well as two concentric walls with several towers surrounding it. The sub keep to the left of the gate consists of a smaller round central keep ringed by a single wall, with towers on each corner. In the distance, a third structure is visible on a mountain top, possibly a second "sub-keep". The the main and "sub-keep" on the other side of the pass are presumably placed so that, even if a besieging army were to breach the main gate, they would find themselves under fire from archers on both sides.
Beyond the heavily fortified pass, there appears to be a city located between the ridge on which the fortress is built and a second, higher ridge.
After it was discovered that King Duran of Elbe was recovering at the JSDF hospital in Alnus (he had previously claimed to be a farmer), the JSDF were able to enter formal diplomatic relations with Elbe, allowing them to transport large forces through the area to attack the Flame Dragon without risking unnecessary conflict. Tubet fortress was the first Elben settlement they arrived at, with Duran arriving by helicopter in advance of the main JSDF forces, much to the surprise of Count Watt, lord of Tubet, who, like many in Elbe, thought Duran to be dead.
After arriving in Tubet, the JSDF forces travelled into the mountains, arriving in time to intervene at the Battle of Mount Tube, slaying the two hybrid offspring of the Flame Dragon led by Giselle before they could attack Yōji Itami and the rest of his party.