In the past, player with TH 16-18 would competitive in one group. Today, there is only one group for all player. It's very hard for players with low TH to earn top prizes
The dragon quest tiers are based on their specific brackets. The brackets are: 1-6, 7-9, 10-12, 13-15, 16-18, 19-22, 23-24, and 25. There are times ATA will put brackets together if they think there isn't enough competition. For example, in early realms since there are normally <25 players between th19-th25, ATA will combine these brackets to try and force some kind of competition. In late realms, th1-18 are normally in the same bracket. In my opinion, players should only compete within their own bracket. It is extremely unfair to force players to compete against players who can attack/gather higher levels. :/ This has been brought up in the past though and I doubt ATA will even consider breaking up brackets again.