Hi yophie, your surfaces are just not close enough to each other to be joined there. Basically some surfaces are flat and some are curved, so they don't match up very well in their shapes and therefore won't join.

Here's a couple of screenshots to illustrate:



If you zoom in to these areas here you can see a fairly substantial gap between the shapes - that's because one shape is flat and one is curved:






In order for those to be joinable they have to be much closer to each other in shape where they meet up, usually that means either they should be created from a common construction curve (not different curves like one a polyline and one a bendy curve), or another possibility for a joinable juncture is to have 2 extended surfaces that are trimmed with each other.

I'll see if I can give you an example of how to construct a joinable result in a minute.

- Michael

Hi yophie, so another problem area is a slight lack of symmetry between these 2 "lobes" making up a section like this:



In order for those edges to match up with each other when they are circular arrayed there should be a tight amount of symmetry in those pieces, it seems that they are slightly different in shape in your original file.

I deleted one of them, then mirrored it to make sure it was symmetrical, then after that I did an array circular and got a better matching base piece which is attached here.

- Michael

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