Fly screens are great for allowing light and cool air into the house while keeping out insects. Security doors have the added bonus of keeping out potential burglars and other intruders.
The frame for many security door is either aluminium or steel. Both of these are quite secure; any thief will need to go to a lot of trouble to cut through metal frames like these. Aluminium is obviously more resistive to corrosion and rust, but under any reasonable conditions the steel security door will not be prone to rusting, and probably last several decades. Salty ocean air is the only possible problem here, yet a galvanized or powdered coated security door will not corrode in sea air.
The mesh or grill on security doors is often metal. It is possible to have light plastic mesh on a door and rely on the frame for security, but light plastic will not last more than a few years. As the security door is designed to last for several decades so it is fitting to have mesh that also lasts this long.
Stainless steel mesh is the best option for security doors, though aluminium mesh is also a viable option.
A door is only as good as its lock and fittings. Good security doors have locks that fit at three points, middle top and base, so the door cannot be bent out of shape.
Hinges on a security door are recessed so that they cannot ever be accessed by potential outside intruders. The best options are either doors with three hinges or a long hinge that runs the length of the door.
A good quality security door belongs on every home. Be safe from intruders, but let the breeze in. Else, close the solid door and keep warm in winter. Enjoy the weather and solid security