Elizabeth,
BoomJ is right about a small pot-metal naval cannon made in Italy. It was made to shoot caps, is all cast and the wheels do not turn. The following pic is an example of the carraige:

I turned a new barrel, .177 - BB gun caliber, in high school no less. My how things change

Yours, however, appears to be made from wood, it is possible someone rebuilt it after removing the metal parts. I haven't seen an example of the capgun out of wood.
There was a plastic kit that is very similar to this style of carraige, a photo of the box:

Popular mechanics had a set of plans of the same naval carraige, which I have, and I combined the toy, plastic model, and plans into this:

Canadian cannon works sells a 1/10th size model with the same kind of carraige that can be seen here:
http://www.brasswithclass.com/carriage_man_of_war.phpI talked to Canadian Cannon works and their carraige is copied from the Popular mechanics plans. For as many examples of this carriage in models, no one has ever come up with a picture, drawing, or any example of a full size example. Very sad 'cause I really like the style. Oh well.
Hope you find this of some use,
MikeR C