Okay, so after a few dozen hours in this map, I decided to make a mob destroyer, a building dedicated to the eradication of Creepers and others. It started with a tower i could shoot from, then i encircled it with cacti. Then I made a massive water way shoving them towards the cacti. Then I made an octagon of fiery netherstone around the entire castle. Of course it is attached to the rest of my kingdom via railroad which currently doesn't work without Minecart Mania, so i've been doing alot of running.
So post pictures of your mob destroying inventions or just comment on others. whatever.
Spiders hop over the fire, then across bridge and thus over water/cacti, and stay out of sight of tower? Guess you could kill them with a sword, but hmm... Is that part of your design solid yet?
But can they open doors?
No they cannot.
I am simply pointing out that if they are bouncing around outside the door they might be in a blind spot, meaning you need to get choppy. Personally I would add a small cactus barrier in front of your bridge, leave 1 space open for you, and spiders will be impaled. Just my bit.