Just lost a fort to one suspended wall...
I'd gotten all the cavern walled off with a single level of fortifications, when I got an announcement about an FB. Aha, I thought, now I can see if it'll spawn in the water or on top of the fortifications. Turns out I'd missed a section and it spawned on land, but that doesn't really matter here. I checked it out -- webber, non-flying. Nasty, but it could be kept away from the fortress while I set up a proper trap for it. I'd modded SILK_TEMPLATE to take no heat damage and have the combat properties of copper, but with silk's usual density (all raw-editable creatures were shunted over to STANDARD_SILK_TEMPLATE, which has the properties of vanilla silk). So I really wanted to catch it, and not hang back and turtle while I handed out crossbows to everyone.
At the time there were two entrances to the cavern, one major entry that linked much of the heavily-flooded cavern together via narrow corridors threaded through the rock, and one semi-abandoned entrance granting access to the southern swathe of the cavern. Before I even unpaused the game announcement, I set into motion my cunning plan. The southern entrance would be sealed where it met the rock, and the northern entrance would be blocked off by rampless channels, plus a few tiles of wall near the lake edges (there were a few rocks nearby, but not enough to cover the whole area. Digging would be quicker). Unlike the southern entrance, the northern entrance couldn't be designated all at once. The walls needed to be built in a certain order, and the deramping would have to wait after the channelling. I'd need to supervise closely, and stop key dwarves from wandering off to stockpile things. Finally there were just a couple of ramps to be dug to stop some stray diagonal access, and I followed the miner anxiously through the last Y-shaped tunnel, watching as she narrowly avoided being tripped and slowed by a stray horse before descending the last staircase.
Wait a minute, I thought. I don't have any horses.
One quick check later, and it was confirmed -- that was no horse. The beast was still in the tunnel linking the two entrances and the fortress, and very close to the fortress itself. But the barracks were near the staircase, and if the creature was engaged on the stairs then perhaps it wouldn't have full use of those webs. Unfortunately, it was really, really fast, and got to the top of the stairs just as the soldiers did. The fortress at this point was a couple of large rooms dug out of a couple of little hillocks on the site, linked by a short subterranean corridor and vaguely resembling an hourglass. The staircase was at the pinch of this hourglass, on the route between the dining room and the food stockpile and within spitting distance of the meeting area. And oh how that FB spat its webs. Thirty dwarves to five, in five to thirty seconds. I expected the slaughter to finish in an instant more, but the FB was preoccupied with a poult too small for it to hit and looked to be locked in place. I wanted to know how it had gotten in, as I'd been watching the only entrance. Thinking back to where I'd initially seen it, it must have come from the south. I trace the path from the tunnel to the caverns... and find a suspended construction across the mouth of the southern entrance. Somebody must have gone to build it, stumbled into one of the scattered GCS webs, and suspended the job with me missing the announcement and being too focused on the north to double-check it.
I have no idea how they managed to find that one quisling web, as they were walking that area when building the fortifications there and the building material was right on top of the construction site. Maybe it was something else, but I have no idea about that either.