Genius 322

[comeragh]

I have completed this crossword but cannot find what I have to highlight. Can anyone help me please? I know the word that unites the categories is WATERLOO but and I quote 'this name - attached to an event that occurred two hundred years ago - occupies one diagonal, which should be shaded'

[fiction-factory]

If I understand you there will be at most 2 diagonals to choose from so you look for consecutive letters (maybe in reverse order) somewhere in there that spell a name relating to Waterloo

[Baldric]

Battle of Waterloo in 1815 maybe?