If you want to provide an extra layer of protection to admin.php, the /install directory, and test & development installations, you can do so with .htaccess authentication.
Protecting admin.php
To protect admin.php, edit the .htaccess file which is in your forum root directory (e.g. /community) and add the following to it:
<Files admin.php>
AuthType Basic
AuthName "ACP"
AuthUserFile "path/to/passwd/file"
Require valid-user
</Files>
The "path/to/passwd/file" will look something like...