I use a similar design to this but with a couple of differences:
I put half slabs down to stop spiders from spawning.
The water channels I make 2 deep or the mobs can jump out of them.
I only make the water channels 1 wide (as I don't get spiders in my system).
This is what my design looks like:
Dark blue is water source blocks. Yellow is half slabs. The black in the middle is where they fall through. Each layer has a 2 high space for mobs to spawn. You could make it 3 high so they can walk over the half slabs which makes it a bit quicker to get them into the water but in the next update you would then get endermen spawn in there which would be a pain.
If you build from around 24 blocks above sea level to the top build limit you can fit maybe 6 or 7 levels on it. 4 levels though will spawn tons of mobs as long as the nearby caves are well lit. Really simple to build and is also very easy to add a water system to flood it and turn the spawning pads off.
I just finished. Missing a few outside blocks on the side (but it doesn't matter to me) anyways I used this design but didn't put any slabs to stop spiders. I like getting string, but I see how it gets backed up. I was looking up to see red eyes climbing the wall in the dark then disappear. After that I looked away then back again and a spider droped down onto the ground and was still alive and began to kill me. I didn't have a sword on me and it was on hard. Haha I ended up swimming to the bottom of the ocean and it disappeared. Thank you everyone for your ideas!
Haha! Yes, spiders occasionally survive the fall. I have a glass catwalk just below my spawner (to lure whatever mobs are just standing there to their deaths) and I can see what's happening. It seems to be a technical glitch in how the spider falls. Once in a long while, instead of a smooth fall, they'll go down a few blocks, pause or even come back up slightly, then fall a few blocks more, pattern repeats. These are the spiders that survive the 27-block fall I have set up for them. I always equip a sword when I go down the stairwell to the death platform because of them. They're almost dead anyway. One hit does them in.
I just finished. Missing a few outside blocks on the side (but it doesn't matter to me) anyways I used this design but didn't put any slabs to stop spiders. I like getting string, but I see how it gets backed up. I was looking up to see red eyes climbing the wall in the dark then disappear. After that I looked away then back again and a spider droped down onto the ground and was still alive and began to kill me. I didn't have a sword on me and it was on hard. Haha I ended up swimming to the bottom of the ocean and it disappeared. Thank you everyone for your ideas!