Dwarves should be able to make cannon as an artillery piece. It would be quite dwarfy, and its perfectly historical for the target time period. (In fact, catapults would likely have been an anachronism by 1400).
Cannon have been in use since pre-1400 in europe. They were used on the Iberian peninsula in the 13th century, and England used its first cannon during the hundred years war at Crecy. The earliest documented use of gunpowder artillery in combat is in China in 1132AD.
The cannon becomes standardized and effective at both siege and anti-infantry roles during the medieval period in europe.
Gunpowder itself goes back to something like the 9th century in China (and was certainly weaponized in Korea and China pre-cannon, so there's something else cool to do)
Dwarves have the metallurgical and engineering know-how to make a cannon.
Cannon make saltpeter a valuable commodity, and give the alchemist's lab a reason to be re-instated (saltpeter -> gunpowder).
Gunpowder could be stored in bags or barrels.
Early cannonballs were shaped stone, although forged metal ones appeared before the end of the medieval period.
The earliest evidence for a handcannon (handheld gunpowder weapon) appears in the middle east in the 13th century, and there is evidence for their use in both europe and the middle east by 1250.
Grapeshot is first reported by 1350.
Sorry if this has been suggested, but the search engine appears to be down. (At least, it keeps telling me it can't access the search daemon).