It turns out that I thought this was the 4804 thread! It all comes of being on holiday and having to tackle two puzzles at the same time when I returned.
I spent a long time solving the clues and obtaining the grid fill for 4804. I think I know what I was meant to illustrate but I had no idea how to do so and found the rubric incomprehensible . So I had essentially done the crossword bit but I did not have the time or energy to finish it.
In desperation I switched to 4805 and managed the grid fill (including the four special answers) only to find myself in a similar position, but this time I was able to discover the 30 cells. Presumably all I have to do is highlight them. However, I haven't got any idea as to how this process is deducible from the rubric.
The irony is that I was on holiday in Rome visiting the various archaeological sites, getting a feel for the 4805 theme! But then KC did not set out to challenge us to guess what he was thinking.