Easiest is to just hide the sheet you don't need, this ensures things doesn't get in the way. You can do more complex selections too, look at this article for more about that:

The break in the wall is a permanent edit (In most cases it will divide the wall into two separate parts.)

If you change your mind right after breaking it, you can always use undo (ctrl+Z), but if you have done other edits in the meantime and undo isn't appropriate, you need to manually edit the wall. In most cases, just using the node edit tool to bring the end node of one end over to the other works good enough, although it might end up leaving a line at the break point due to the fills not lining up seamlessly.

Other options include just drawing a small wall-piece into the hole, or just simply delete the existing wall and just redraw it. That's usually quick using a wall drawing tool.

You have the tools from the wrong style open. If you look at the dropdown in the dialog, it is set to Annual Realms of Legend, not Annual Dark Realms. Just change it to the correct one.

You need to move the floors back to the BACKGROUND (FLOOR 1) layer too. The corridor tool operates on both floors and walls, and expects both to be correct for it to recognize an existing entity in your map as a dungeon room/corridor to work with.

Hmm... Old maps should normally work fine as long as you have the same add-ons as you did when making the map. I can see there are some spots on that map where the background shines through, so that is loaded just fine, but obviously there is a lot of things missing. Do you know what art you used when creating that map. Was it standard CC3/DD3 art, or perhaps you used some 3rd party art like the CSUAC package?

You can gather clues from the file names. You can find symbol filenames by opening the symbol manager, picking a symbol and hitting the lsit button, while fill style file names can be found on the bitmap files tab on the fill style dialog. If you post the .fcw, I should also be able to tell you pretty quick.

As for your CC3 dungeon question. CC3 Dungeon is just a very limited dungeon style, more like a demo than a proper full style, only allowing you to make rather simple dungeons. As such, it doesn't have a lot of symbols, and all the symbols are already loaded in the symbol catalog window, the catalog buttons at the top aren't connected to the style at all, and just load catalogs from a more or less random style.

The reason for the bad resizing behavior is that you have lots of things on your MAP BORDER layer that doesn't belong there. There's a lot of buildings there, and most of the city wall is there too. This confuses CC3+.

To fix it, show just the BUILDINGS sheet, then use the command Change House Layer on all the buildings, and change it to BLDNG (HOUSE 1). Then make a new layer, call it WALLS. Now show only the SYMBOLS 2 Sheet, use change properties on everything and set it to the WALLS layer. Do the same thing for SYMBOLS 3 sheet.

Now, the resize command should work normally.

You probably have attach enabled (button lower right)

Was the original polygon properly closed? In CC3+, it isn't a polygon unless it is a closed shape (it's then a path), and if you try to set a fill style for a path, it is only applied to the line itself (And keep in mind, in a CAD based software like CC3+, being closed is a property of the entity, not just that it looks closed on visual inspection, there is a close path command by right clicking the explode button).

Your map is nice, but I would recommend you look a bit at your grass, the repetition of the texture is a bit much. This can often be fixed by adding more stuff to the map, combining it with other grass/terrain textures, or changing the resolution of it so it doesn't repeat quite as much. Also, maybe not use the same floor style for all the rooms. You can also use a shaded polygon to angle the fill in that stand-alone building to fit the direction of the building.

Since CC3+ always tries to open the last map on load, there is a chance that something has gone wrong with it.

First, to open CC3+, try to either double-click on another .fcw file on your drive, since that will open CC3+ with that file instead of your map, or rename your map file before starting CC3+, since this will cause CC3+ to not find it and will open up without a map (showing a black screen).

After that, you can try opening your map using File -> Open. If that crashes CC3+ immediately we know where the problem lies and can take it from there.

In that case, I think it is best that you put in a tech support request. You'll find the form on your registered user pages. They should be able to get you back up and running.