So now to build vertical walls from the above base piece which will be joinable, use the edge curves of the base piece like this and and extrude those same edge curves upwards:






These pieces will be joinable because the surfaces were all constructed from a common curve, that's the key thing...

To make the top flat you can draw in a line in the Front view and then use Edit > Trim to trim end off the extrusion.

Hope this makes sense - basically you will get into trouble joining things if you try to use different curves to construct things where pieces are meeting up, you've got to either use the same curves or use intersections between surfaces in order for things to match closely enough to be joinable.

- Michael