JustObserverSo, are we talking about following the rules or following Dёti's suggestion? According to the rules, you can still build a Grand Mage's Tower in two castles if you don't have the required level.

If you don't have the required level, then of course, you should build a Grand Mage's Tower in two castles!

Added 2 minutes later

Here's an interesting point: let's say two top players or just two players who are friends and want to go further make it to the top 4. They agree not to attack each other but to eliminate the other two, or even one of them will do it, while the other just waits for them to be eliminated. Is this a good thing?

You can come up with anything! You can agree with players in advance who to eliminate and who not to... you should also think about whether it's worth attacking or not... there are many options! It's not a fact that you'll agree to be friends with someone, and one player with a powerful army won't overwhelm you... and it won't be that easy to be friends here, because everyone plays for themselves... and in the final, only one winner will remain, not two. So, you should think a lot before each game