I never really bothered to seal off the 2nd and 3rd carven back when i discovered them, since i had defences higher up and when i discovered them there was little activity in that area. Years pass, and the depths become busy as i constructed magma forges and started my(still ongoing) attempt to drain the bottom carven layer to better get at Adamantine. As my fortress wealth increased i was attacked by increasing numbers of forgotten beasts, but i never bothered to build any defences lower down as the handful of l steel armed and armoured legendary melee dwarfs, who composed the core of my military(other than that i only have a few lacklustre marksdwaves and a lot of very recently recruited peasants), were more than capable of forgotten beasts, as they would go on to prove numerous times.
That is, until a web-spewing crystal glass humanoid showed up on my second carven layer, from which it had complete access to my main stairway. This did not worry me at first and i sent my legendary squad down from the surface to face it, however when it was about halfway to my stairway i remembered how in the early days of my fortress a web-spewing titan had attacked and wiped out most of military before proceeding to kill two dozen random civilians before being punched by some random nobody. I quickly ordered the stairwell to be walled but by that point it was two late and the Forgotten Beast reached the stairwell at the first time as the first of the workers. The results were terrifying

A legendary swordmaster with years of experience and over twenty kills, clad in high quality steel, was webbed and killed before he even got a chance to attack, A legendary Macedwarf and a baroness(who was more importantly a Legendary Brewer) guickly followed. I then watched in horror as the Monster proceeded to kill everyone who was walking up or down the stairway in that area, eventually killing 32 more dwarfs(including a Legendary Brewer and Miner who had been a backbone of my fort), before eventually being struck down by another Legendary Miner. It looked pretty horrible, and many of fortress denizens now suffer from a vary high degree of stress. I in the process of building another graveyard to hold the bodys as well as working out how to seal off the(pretty much useless) 2nd carven.
P.S. How do you post images