Clash of Clans features many game modes with different matchmaking systems. Here are all of them explained.
Above 5.000 Trophies you will join the Legend League that features a completely new matchmaking system.
Since each player can intentionally gain or lose Trophies as they desire, you essentially have control over how hard you want your opponents to be. This means that a level 11 Town Hall can come and ruin your day if he wants, just because he ditched Trophies and ended up with the same Trophy count as you.
However, the game wants you to play against player with the same Town Hall level, this is why there is a loot penalty for attacking lower Town Hall levels. You can find more information about this on the Loot Calculation page.
Unlike the regular multiplayer, it’s not about trophies this time. The game calculates how "strong" the villages on your clan are and then looks for a clan similar to yours, so the battle should be always even.
This is not as easy as finding a village with similar Trophies, so the matchmaking can take longer. Fortunately, there is no need to wait in the Clan Wars screen. You can close the screen with the red "X" button and you will be notified when an opposing clan has been found.
Another difference from multiplayer: you can’t choose your opponent, you can’t skip. As soon as the game finds a Clan similar to yours, the Preparation Day begins.
Over time many techniques were created by the clashers to take legally advantage of this system. Town Hall 8.5/9.5, defenseless bases and adding a couple of very weak bases can help to get an easier war, but they are no longer really useful.
The matchmaking is based only on the current league your clan is. It's a ladder-based war. There is no war-weight, Town Hall levels, Engineered Bases, nothing, only the War League your clan is matters, making this a truly skill-based game mode.
One quick note, the first time you enter on a CWL your clan will automatically enter a specific league. This league is based on the 15 strongest players of your clan. The higher your league is, stronger your clan and enemies should be.
On the first CWL event there were many complains of unfair matchmaking, but as soon as the stronger clans level up, the battles will start to be fair. It will take some time, but eventually you will be match against clans just as good as your clan.