The regions with the most top castles

Hover with your mouse over the thumbnail to enlarge the picture. The pictures and the regions link to the list of top castles in that region.
Query failed : Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'topkastelen_nl_-_topkastelen.castles.country' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by