Newbie question, joining line spline with arc spline?

[Dystopiian]

Hello! I'm hoping someone could help me.

I'm a Uni student learning 3ds max for games development and we're doing a floor plan assignment, I am able to create simple box shapes with the spline tool and extrude walls from that, however, I'm creating a somewhat unique floor plan which involves an arc (that I'm planning to be used as windows) and when drawing out my plan I do not get the dialog window "Close this spline?" I've tried connecting all the verts manually (don't understand how to weld yet) and still no luck.
I was able to attach the inner and outer splines together and extrude walls from them but the walls are not solid/filled. They appear to have space between each other. I think this is what's stopping me from being able to use the Boolean tool to create windows and doorways. 

I will attach a screenshot of what I'm working on, if you need better screenshots to see whats going on I can happily upload more.

In short: Able to attach, extrude splines but without closing the spline because I used an arc spline. Walls are 'hollow' as it were. 

[KarlAkimor · Accepted solution]

There are several aspects to weld verts:

You only can weld a vert to two segments, anything else use simply fuse.

If you are doing splines they need to be attached in order to weld them

You can avoid creating new lines deactivating "start new shape"

You can activate the snap to verts to join different lines

For welding overlapping verts simply select them and use weld in vert subobject mode in the edit spline

Notice in the menu how it says how many verts you have selected, the weld menu and the distance that will allow weld verts and the automatic weld system.

You need to use shell instead extrude.

[Dystopiian]

Karl... words cannot describe how grateful I am about your response. Thank you so so so much. 
I did have to start again because I'm pretty sure I passed the point of no return with my the file I had in the screenshots. 

You even taught me some things my teacher hasn't taught us yet. 
And it still took some trial and error on my end as I'm not used to all the tools and interface yet so it felt awesome to trouble shoot that with your help! 😄

But I did follow your instructions step by step and a weight had been lifted. Thank you!
Can i ask, is there a benefit of using Shell instead of Extrude?

 (I know I'm using Extrude in the pic, I was just testing it :))

[KarlAkimor]

Unlike the Extrude modifier, you can extrude faces in both directions simultaneously (up and down). What’s more, you can add material ID settings to the top, bottom, and edge polygons directly from the modifier.

And finally, you can control the edge profile with another spline– just like the Bevel Modifier. So the Shell Modifier is really a combination of Extrude and Bevel Profile.

[KarlAkimor]

The advantage of extrude is that you can extrude open splines (it won't be a solid object) when with shell they need to be closed.