Cusomizing category page

Hi all,
let’s assume, that i’ve got the following structure of categories:
A
B
C
D

We all know how does it look like. In addition – these categories are also set as a menu. However – after clicking into (let’s say) category ‘C’ I’d like to see (inestead of the category posts) a page with next menu – just links to categories below. It should look like this:
C-> C1, C2, C3, C4, …, Cn
And after clicking into one of the C’s i’ll get posts from that sub-category.
Thanks in advance for any help,
Mike