I use Tiled to make maps myself. The C2 editor is very limited and Tiled has a lot more tools, then you can save and import into the Tilemap using the "Import TMX to this object" button in the Tilemap window. (You can also export C2 tilemaps)
In Tiled you can select all the tiles you want to be solid using the "Select same tile" tool while holding the shift key. Then create a new layer and paint the selected area with say, tile number 1. Then you can import this layer into the solid Tilemap