What was the highest attendance of the matches that took place at Windy Hill?
CREATE TABLE "round_15" (
  "home_team" text,
  "home_team_score" text,
  "away_team" text,
  "away_team_score" text,
  "venue" text,
  "crowd" real,
  "date" text
);
SELECT MAX("crowd") FROM "round_15" WHERE "venue"='windy hill';