I guess they're not gaps in the wall as there aren't walls; I mean remove the space underneath the platforms (seen in the top left of your screenshot) and just make it into a fat, solid wall. Stick in a portcullis, and if you can add more hay, stables, and don't make it so "boxy". Putting 5 or so explosive barrels next to each other is also a bad idea...

Hai - Thx for the comment sorry for putting the file twice.
But which huge gaps in the wall and which physics/ explosion ?

Er, why have you put your .BSP in there twice? It unecessarily makes the file size larger! Nice concept, but you've still got a long way to go- make it look like a real castle (i.e. remove the huge gaps in the wall), use displacement surfaces for the ground, don't have physics/explosions in there just for the sake of it, etc. etc...