Hi Jiro, It looks like you are making good progress now.

Yes, I can see the part where you had pieces that were kind of hanging off one another, it is pretty easy in that situation to have the "hanging off" piece not be a completely closed loop on its own.

If you want to have stuff like that all at the same level, then you may want to use the Edit/Trim tool to kind of cut away the interior connecting piece and have just one bigger outline.

Like for instance here is a curve that is hanging off another curve:



If you want this all to extrude as one block, then you can select the one that it is hanging off (shown selected in yellow above), and then run the Edit/Trim command.

Then pick the right-side piece as the cutting object and push Done (or right-click which is a shortcut for Done).

That will slice up the first curve into pieces and you can select this piece:



And then push Done or right-click to remove that piece:




Then once those pieces are touching you can use extrude to build one single block out of it.

- Michael