Rotunda 18th C light garrison gun

The style of barrel looks 18th c., or earlier, but garrison guns in the muzzle-loading era were kept in service for many years. The carriage, with its adjustable height, looks more 19th c.
Garrison guns were used in fixed fortifications, like coastal defence forts, so their carriages were not designed to be very manoeuvreable and they sometimes had adaptations, like this one, to allow them to shoot downwards onto attacking forces.