admin URL

You can't have assets in the /system or /application dirs. Those aren't public dirs and are protected. Just create a new dir off your root to hold your admin assets. Here is what I typically use

/web_root
--/application
--/system (/system is usually outside of web_root)
--/assets
----/frontend
------/css
------/js
------/img
----/admin
------/css
------/js
------/img

Load a frontend asset:
<link href="<?php echo base_url('assets/frontend/css/bootstrap.css'); ?>" rel="stylesheet">

Load an admin asset:
<link href="<?php echo base_url('assets/admin/css/bootstrap.css'); ?>" rel="stylesheet">