since I am frequently attacked when I call realms "B-tier" or "C-tier" as if I'd attack their honor with that description, I decided to write down how active and experienced a realm should be - in my mind - to qualify for the different tier levels.
Leaders: Have to be very experienced and must be willing to invest an absurd amount of time.
Members: Most of them are very experienced and several of them are able to lead realms themselves (=> realm could successfully counter even big attacks themselves in the unlikely event that no leader is present, etc). Most accounts also have two or more players, ensuring they are available at least 20h/day (for defense and offense)
Leaders: Have to be experienced and must be willing to invest a lot of time.
Members: Many experienced players in their ranks. Able to place towers effeciently and quickly. (That means if they see that at 4am 10 new towers can be placed, someone oranizes that this actually happens)
More than 2/3 of the members are very active and available (at least for defensive purposes) ~18 hours / day.
Also most supplies are cleaned within 10-20 minutes of their opening. (That means the warlords are already waiting in the tower next to it and are not being cleaned from the ark)
Member consequently use only 1 WL to conquer goblin-towers.
Troop-supplies are donated to the max lvl within a day.
Training Grounds are highly regulated and give the max XP in as short a time as possible.
When they go to war every single member has the appropriate MPs skilled for it.
Leaders: Experienced (played at least once in a B-Tier or higher realm as a member) and is willing to invest a good amount of time and effort to coordinate his group.
Members: Many experienced players but they don't take the game too seriously. They build towers when they are online but they wouldn't intentionally stay awake 1 hour to reduce time-loss. Also only some in the realm send their WLs to the towers that open up the supplies, the majority cleans them from the ark. Same with goblin towers and additionally to cleaning them from the ark more than half their members wouldn't know the time when their daily respawn takes place. The player know that supplies are more important than their island, but more than half of them have the focus on their island regardless. In an offensive war they have at least 5 players who barely contribute. Defensively they avoid the biggest mistakes.
TG wise they know what to do and most/all players use the correct warlord with the appropriate runes.
2-5 members don't have the correct MPs skilled.
Leaders: Has at least played one round prior to this. Knows in theory some stuff his members do wrong, but also knows that his members are not active enough or willing enough to change their behavior. Tries to educate them regardless and he is happy when 5-8 of his members actually do what he says.
Members: Most of them login at least once a day. Offensively many of them aren't active or willing enough to attack twice an opponent twice a day. Supplies are cleaned with whichever troops are available. Goblintowers are cleaned with 2-4 WLs in the same attack. Not everyone in the realm uses the correct WL for the TG and nobody cares / no consequences for those players. Towers are frequently misplaced and either need to be fixed or just stay the way they are, even though a litte effort would free up a lot of towers with no loss of supplies.
More than 5 members have questionable MP skilled.
Everything below C-Tier. The majority of members don't care what happens ingame.