I found a spider spawner very close to my underground base (the base has now expanded to include it, and I'm trying to convert it into a poor man's XP farm / library).
I originally wanted to make the farm such that the spiders fell down an 18 block drop, but then I realized that waiting down there for them to drop was so far away that it turned off the spawner. So I compromised and made a 12 block drop that doesn't put them quite on the brink of death, but still makes them a little easier to kill.
I tried to make it so that the spiders are unable to crawl back up the wall (and subsequently fall back down and kill themselves), but I haven't had much luck - they somehow manage to crawl up the inside wall right above the fence posts. There aren't as many spiders committing suicide now, but I still lose all of the ones that choose to crawl up the one side of the wall.
In the future, I'd like to ideally find a better XP farm when I find different mob spawners (working with spiders has been annoying), but for the time being I have to make do with this. It works somewhat well if I go AFK for a while, but if I'm actively hacking away, it's a slow influx of XP. Basically, I'd like to know what I can do to get this thing operating at peak efficiency. Do you guys have any input/suggestions you could offer? Thanks
I put cactus in the walls of my spider farm. I forget how far my drop is, but probably similar to yours (it gets them to a 1 hit kill with a sword). With my farm I can also AFK to get string (they drop down, climb back up and get killed by the cactus).
Kill/collection area looks like this (there are hoppers under the carpet).
BTW, dungeon spawners are not a good way to collect XP. It's always a slow process. I use them early game until I build something better, then I don't use them after that. In my older world that I have been playing since version 1.8 I have a skele grinder and a spider grinder (the one in the pics above), but since I built my guardian farm I don't use them any more (I also found a zombie dungeon, but I haven't really done much with it).
In my newer world that I started in version 1.10.2 I have a zombie grinder that I used a lot early in the game, but since reaching The End I don't really need it. It's faster to stand under a 2 tall platform, look at a bunch of Endermen, and whack 'em with my sweeping edge sword. I get lots of XP and ender pearls pretty quickly. Not as fast as a proper enderman grinder, but fast enough for my needs. Actually, even before reaching The End it was faster to just snipe mobs from my tower at night, then collect all the XP and loot after sleeping.
You can also get a lot of XP and other items by AKF fishing.