雲橋　[うんきょう]　lit. "cloud bridge," a siege tower
叢雲橋　[そううん]　lit. "storm front bridge," a really big siege tower (I call it a "thunder bridge," short for "thunderstorm bridge")

The chapter describes three basic types of military vehicles: the smaller assault wagon (戦車 lit. "tank" or armored personnel carrier), the siege wagon (填壕車 lit. "trench warfare vehicle"), and the siege tower. Siege towers (or "bridges") come in two varieties: big (雲橋) and bigger (叢雲橋). Based on the literal kanji readings, the siege wagons would be used to breach the trenchworks that constituted an castle's outer defenses, and then would haul the siege towers up to the castle walls. The "bridge" kanji (橋) suggests they would be tall enough to deliver troops directly onto the wall walks. The anime seems to only depict the siege wagons.

This is model of a traditional medieval European siege tower, but I think it approximates the form and function of the "cloud" and "thunder" bridges.