That's a very interesting weapon, I'll call it a howitzer due to the exterior, but I don't know what it is technically since I can't see inside, I don't think I've seen another piece just like it.
The royal cypher on the chase appears to be that of Christian 5, King af Denmark · Norway 1670-99, so we can date the piece fairly accurately without knowing what the rest of the marks are. Look at how far forward the trunnions are.