Alliance wide defense travian strategy work great. This need a team of players who work together, this can be implemented by sitter checking.

When a player get attacked, he will contact the leader. Leader will send an MM asking all players to send reinforcement.

ALLIANCE A

Lets say the alliance got 60 players. Most of them sent say 100 to 500 troops as reinforcement and you got 10k defense. Now you get the attack from hammer with 40k troops. All defense get killed. Players are afraid to act on defense calls.

ALLIANCE B

You have 40 players, all active. Any player who do not help other players will get kicked from the alliance. Everyone have 2000 defensive troops. On defense all, all players sent all their defensive troops. You got 80k defensive troops with many heroes. Defender have level 20 wall, adding extra defense bonus. Now attacker hit with 40k troops, he lost all of them. He will never attack the alliance members.