This is the strongest mooring if there are no rings or bollards. You can get special bent hooks that go behind the Armco, often called "Nappy Pins" because of their shape, or you can use chains.
If you do end up mooring on pins, angle them so that the top of the pin lays towards the ground i.e. not at 90 degrees to the ground and place the pins outside the length of the boat. This will help to stop the boat swinging backwards and forward when a boat comes by. Best not to tie a center rope off the roof too.