I have participated in several worlds and the usual form of game is that when an alliance, in fighting against others, gets to pull the fortress usually helps the other alliances (allies) to achieve their goal, destroy the fortress.
This alliance can then help its allies to destroy the fortress and thus recursively.
In some cases I have seen as alliances that were enemies have also been helped to destroy the fort and obtain their medal, although in this case I did not seem to me an adequate solution. If they have been marked as "enemy" it has not been on a whim, rather because they have done very damaging actions against our alliance.
In this case it is normal that the alliances that are destroying the center continuously leave aside those who are their enemies and do not give them access to the fort.
In this case of disparity of forces it is better to resort to diplomacy and get an agreement with the larger or dominant alliances in the game. If there is resentment among the alliances, you will never find an adequate solution but with a good diplomatic work it is possible, and I repeat that only possible, to reach an agreement even between enemies.
