Human traders and diplomats will walk right over the pressure plates without setting them off, walk over the burning lignite and catch on fire, then run around the map setting everything on fire as they do.
Cats and other small pets are also too small to trigger pressure plates and will also go down the burning corridor.
Trolls and other buildingdestroyers will smash your floor hatches and make the entire system useless.
I thought diplomats and traders were always considered "dwarves" as far as pressure plates go? The first set of pressure plates should redirect them down the "safe" path.
Cat pets should trigger the friendly pressure plates as well, if they are set for minimum to maximum weights.
Trolls might smash some of the hatches and whatnot, but I don't think they will smash burning lignite floor grates. If they smash the normally closed magma floodgates, well, it will make a mess, but still do what it's intended to do - release enough magma to coat the floors and set everything on fire.
However it might be a good idea to include some sort of manually operated drawbridge that will prevent access to the innermost defenses
Additionally a couple tightly closed but not forbidden doors around the burning floors might be good as well.
Looks like I need a couple more normally open floodgates in the safe passage at each end, and a reversal of the plate and hatch assembly at the fortress (right) side of the trap system.
Another thing to consider - the burning lignite floor grates - will they allow loot to fall through, or will anything caught in that passageway that doesn't burn or melt be inaccessable forever? If the loot won't fall to the lower level then I'll need to reconsider, and just make the thing a weapon-trap and water/magma based defense.
My goal here is maximum protection with minimal dwarf input. I have never been able to make any military work, and have entirely given up on anything military in the current DF versions. I do want access to the outside world though, for trade.
I'll fiddle a bit with the design later tonight. - Excel is your dwarf fortress friend, and I forgot to send myself the file
Oh, also, burning lignite has no melting point I thought, and would burn forever, even underwater? I've never actually used burning lignite floor grates, but think they would be wonderful for kobold thieves and goblin snatchers?