Sorry about the issue with getting poisoned sometimes, I haven't found a way to make that not happen without making the farm super awkward (such as using the quarter block hole in stairs to block them, which sort of works if you want to try). Let me know if you find a better way to contain them since they are smaller than normal spiders and thus most methods don't work. Hopefully the farm is still useful Edit: I should specify that the ladders also make it more complicated. If you don't care about cramming then you can use slabs and trapdoors (to get less than 1/2 of a block) to contain them which may work better (not tested)
Using a different block will make it harder for the spiders to fit into the blocks to either side with the ladders. They would fit but a lot would try to climb on the middle block and get stuck there. It would probably work still but it's more clunky
![Cave Spider String Farm! ▫ Minecraft Survival Guide S3 ▫ Tutorial Let's Play [Ep.46]](http://i.ytimg.com/vi/zmcAHkIaGUE/mqdefault.jpg)
Sorry about the issue with getting poisoned sometimes, I haven't found a way to make that not happen without making the farm super awkward (such as using the quarter block hole in stairs to block them, which sort of works if you want to try). Let me know if you find a better way to contain them since they are smaller than normal spiders and thus most methods don't work. Hopefully the farm is still useful
Edit: I should specify that the ladders also make it more complicated. If you don't care about cramming then you can use slabs and trapdoors (to get less than 1/2 of a block) to contain them which may work better (not tested)
If you don't want the iron door to open, why do you use a door and not a different block?
Using a different block will make it harder for the spiders to fit into the blocks to either side with the ladders. They would fit but a lot would try to climb on the middle block and get stuck there. It would probably work still but it's more clunky