I need the "Main Categories" should have their own landing pages. For example, I am clicking on "Category 1" then it should take me to the landing page for categoriy 1 and should have its own design accordingly. Think of it as a "home page" for category 1.
As marius said, in order to customize a particular category, almost all things you can set up via admin itself.
For this go to
Catalog > Manage Categories > [select category you need to customize]
Here you can see a
Design tab, which is used to customize the category through layout updates.
Now if you need to do this via
layout XML File, then there is another provision to do this.
For each category, magento generates a unique layout handle. So apply your custom design update to this layout handle. The general format of this unique layout handle is
ID is the category id of a category.