You can do this using cleave surface task as the following steps;

Tools->Surface Builder->Cleave Surface->select cleave plane (211) -> cleave

For more details, you can see the following link:

https://www.researchgate.net/post/Do_you_know_how_to_describe_a_mulit-leyers_in_CASTEP

For more details, you can see the following link:

https://www.researchgate.net/post/Do_you_know_how_to_describe_a_mulit-leyers_in_CASTEP

When you are cleave surface, it is important add the vacuum to the slab

I agree with Moaid k Hussain

Thank to Osama Thamer Hassan

I agree with Moaid k Hussain

There's a video tutorial for it here:

https://youtube.com/playlist?list=PLkT2IxkNltgMrtxdGHh_0iGOvousUq12X