Castle Over-Run
It's not right that a castle with 101 defenders can hold out against 10s of thousands of attackers. It's also wasting the game with players using trickle defence to hold back 10s of thousands of attackers.
There should be a ratio threshold that once reached the castle should be Over-Run and captured. e.g. Castle would be over-run if 50000 attackers attacked a castle with only 101 defenders.
That threshold should relate to the castle fortifications level, the higher the level the higher the threshold. Also it should only come in to play if the castle troop numbers fall below a certain number.
I would suggest a ratio of 10 to 1 for a fortification level 1 rising to 50 to 1 for level 20 forts. With the caveat that this only applies with castles with less than 300 defenders.
any castle with more than 300 defenders would have battles calculated the same as it is now
Any castle with less than 300 defenders would have battles calculated the same as it is now unless the threshold was passed in that case the attacker would over-run the castle.
So a Level 20 fort with 100 defenders would be over-run if 5000 attackers attacked at the same time. A level 1 fort with 100 defenders would be over-run if 1000 attackers attacked at the same time.