This isn't anything too fancy, and is sort of a rehash. Also, it isn't meant to be the sort of trap you just idle in by a dungeon. I suppose it might work if you stagger blocks to route the current, but I haven't bothered testing it out. The idea here is to draw monsters to your trap, not to build it under a dungeon. There is no top view, because you don't need one. You just build either of these in a wide band.
I tend to build a series of catwalks above the ground in the area of the trap. The space between the ground and the bottom of the catway blocks is 4. They're made of mostly cobble on the bottom, with glass rails and occasional glass portholes so you can see what's under you. The rails keep skeletons from hitting you much, and obviously make it harder to fall off. I make them in a grid pattern, with ~15-25 blocks between intersections, and a single torch on a block another 6 or 7 squares above each intersection. This keeps a bit of light on the catwalk to cut down on spawns, but doesn't affect the ground. If you get mobs on the catwalk, you just lead them to a corner and they fall below. The catwalk should anchor in the center of your water trap. That way the mobs sort of spread out when they reach it and drown good and proper like.
Cross section of my catwalks:
*Once in a while, you'll rile up a skeleton who will be firing arrows at you. If you happen to have a creeper chasing you and the skele hits him, you will get part of your catwalk blows up. At this height, usually enough survives so you can jump over the missing blocks if you don't have time to repair them.
You lure them in, then go down and watch them die as all mobs should. All of their goodies spill out into a little raceway, and you can run along picking up the goods!
These are cross sections of the trap. The mobs chase you, fall into the deeper water, and will eventually sink down and get trapped. You just run in off your catwalk and cruise down some steps or whatever. You could put a door right in the face of the glass wall too, but spiders are faster than you. Plus you'd need to make the wall one higher so those same spiders don't jump over.
Standard trap: (You might want the glass 3-tall above the water so mobs don't stack and climb over, though this is pretty rare)
  
  
My fancy trap: Don't try to take out any of the depth, as this setup allows you to wrangle mobs, then go out for more before these even die. If it's any more shallow, they can fight the current and sneak back up to air. It has to be two-deep at the bottom!
  
  
  
= You have to leave that as dirt until after the trap's waterfall is working. Once it's running, you can dig it out and then fill in one corner with a bucket of water. It'll fill the rest in automatically. The water doesn't flow correctly otherwise.
= This one has to be dug out AFTER the water is flowing. If not, it'll leave an air bubble where the glass meets and spiders won't drown.
= Usually the 2nd story of my trap. It allows for enough glass so Skeletons can't shoot you, but you can jump up to catch any mobs' attention.
*I also leave two spaces for item pickup to keep some space from creepers. They shouldn't blow, and didn't when I tested it out, but it's scary!
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"If you demand realism, then why are you playing a game? Go outside." -Movajo