Well not Fort Knox - enough to deter the casual opportunist. If I need extra while I am away I can always add a padlock
then you could fit a mortice lock with a (fairly) long key. It needs to be half the thickness of the door, plus the thickness of the lock. So about five inches plus the bow, which is manageable (presumably it will be on a hook by the back door, rather than on your key ring every day).
A low cost one to deter casual opportunists is a Fire Brigade lock, such as an FB2
https://www.ebay.co.uk/itm/London-F...600067?hash=item362aff2c43:g:ICYAAOSwdb5d1mUj
You can buy spare keys, including long ones like in my pic, very easily. If you want, you can buy an identical lock for your shed or side gate so you only need one key.
On my gates I use a Chubb (now rebranded Union) mortice lock, these have brass and stainless working parts so corrosion resistant, with a heavyweight steel case. You can clean and lubricate them every ten years, and repaint round the keyhole where the key tends to scratch the paint off. In a thick gate like yours rain will not reach it, though it may get damp, and you need a large keyhole cover on each side to prevent the wind blowing through, carrying dirt and grit.
They are expensive to buy new, but readily available used on ebay, and simple to dismantle and service. As you will be getting copy keys cut with the longer shaft, you don't need one with several keys (which sell for more).
Off-the-shelf mortice locks sold in hardware stores are not of such high quality or as durable, and long key blanks may not be available.
