Hi Kwin,

re:
> but I still don't understand why without this arc, I can't create solid.

I would need to see the .3dm model file containing the problem object to be able to give you some more information on this part.

On the original file you posted above, "MoI Hill.3dm", you had 2 planar sides already built and I am able to do a network without the extra arc and then join those together and then seal off the bottom using Construct > Planar to make a solid.


That looks like a cool area for kids to use at school!

- Michael