try to destroy the mortar ammunition crate (it's either outside the castle near the radio jammer or near the radio tower at the south part of the map) as it lowers the number of enemies by 3 which is not much but it can definitely help).
If you destroy the AA emplacement (which is wise to do before the radio jammer inside the castle), your commander drops a few 8x paratroopers so that you have a noticeable boost of allies until the over capacity is killed by the enemy. Seized this opportunity to push like crazy. For myself I try to cut the reinforcement of the enemies at the eastern part of this castle "islands", if I manage to do so, my allies might have enough power to take the castle base and then you proceed to the next phase which is easier.
With a vest and a riot shield you might achieve to enter the castle even before you destroyed the AA. Don't forget the C4 though, it happened to me more than once that I managed to enter the castle and realizing it was all for nothing as I don't have C4 on me
You could also try to rush to the castle entrance with the humvee. With a bit of luck you might do it, I did already myself.