Considering the map variety as of now, it seems like things are heavily biased towards the defending team. What I think would be a good remedy for this would be to have two rounds: The first round would be to set a goal for the second team to beat.
If the Defending team successfully defends in the first round
Factions will switch positions and the defenders of the first round will now be on the attacking team. They will need to remove more flags than the previous attackers did last round to win the match
If the Attackers win the first round
A new time limit will be set based on how quickly the attackers overrun the defenders. The new attackers must capture the enemy castle faster than the previous attackers did.
If the Defending team successfully defends in the first round
Factions will switch positions and the defenders of the first round will now be on the attacking team. They will need to remove more flags than the previous attackers did last round to win the match
If the Attackers win the first round
A new time limit will be set based on how quickly the attackers overrun the defenders. The new attackers must capture the enemy castle faster than the previous attackers did.