Guess what! I finally beat Costelmark Tower menace. This is no mean feat, believe me. I had completed all the other dungeons in Final Fantasy XV, including Pitioss Ruin (which my teen did for me as part of the 'Stealing the Past' quest, as it was a puzzle one and I'm hopeless at that, I just like bashing things with my sword). It's definitely worth the effort of doing Pitioss Ruin for the Black Hood accessory that is available for Noctis only as the prize. This came in very helpful in the Costelmark Menace (all menaces are available after you finish the game for the first time).
There is a great guide here giving hints and tips on how to set your characters up and get through Costelmark Tower inner dungeon. I definitely recommend it (you'll even spot my comments near the end).
Prompto's pic of us all in Magitek Exosuits
If you have the Magitek Exosuits (see my pic above), put them on before you head off at camp 5 and you'll be invincible for the rest of the dungeon. The exosuits last for 30 minutes but take 20 hours to recharge, so go for it big time!
My pic of the Costelmark menance
I can't tell you how happy I am. This means I have completed everything on the original game and the extras after Chapter 15...well, except for getting Noctis to a Level 10 fisherman. Level 5 was all my boredom threshold could stand *grin*