For safety purposes make sure your mortise locks are at least five levers.  Nothing less will do.  In fact, many home insurers now insist on five lever locks, and for good reason too.

The best door chains are those that have right-angled plates that fit into the side of the rebate and turn round the edge of the frame.  They are almost impossible to force open.  You can also strengthen your door chains by using longer screws than the ones supplied with the chain.

It is best to fit a mortise lock a bit below the middle horizontal rail.  If you fit any higher and close to the middle rail you will have to drill into the joint of the rail.  This will compromise the security of the door.

You can improve the security of external doors by fitting reinforcements to the door and frame. The reinforcement prevents a door from being kicked open.  They do not require a lot of work to fit, and are quite affordable.  There is no excuse not to get one.