How to subdivide keeping the form of the curve_

I've made the modeling of an object, and now I would like so subdivide it. When I go to edit mode, Edge, subdivide. It creates the line between the two nearest vertex but all in the same surface; I would like it to keep the form, to add resolution.

Before subdivide:

After subdivide:

Desired result:

I would like to make the curve smoother. In this case this "curve" is paralell to the grid, but If I want to make the same with a "curve" that varies in a 3D way, I understand that the new vertices should be along some normals?

PD: I want to increase the poligons because I would like to 3D print the object so I need a "smooth" surface.

Thanks in advance

Edit 1:This is the model I'm trying to model. I know that the topology specially on the little door is very bad.

If I add the subdivison surface modifier this happens:

Modifier:

Result:

The lower edge gets cramped end veri far from wireframe and the cannon...