You can't access the application folder directly.
EDIT
You CAN access the application folder directly, but you really shouldn't. It's not safe because it allows direct access to your logic files (controllers, models, etc.). Refer to this question: CodeIgniter + CSS
Stick your assets folder outside of application and add it to your .htaccess file:
RewriteCond $1 !^(assets|other_toplevel_folders)