I suggest you build the trap underground. This is easier than having to build a really tall tower (and risk falling yourself). Dig a really deep hole and at the top of it build a large room that is 2 blocks high. Make sure it's large and has no windows or torches. The only tricky part about this is being able to co-ordinate the place where the monsters fall and the opening where you are going to collect the goodies.
This trap will kill 100% of all the mobs as long as you dig the hole deep enough. You can even add some cactus to the bottom of your shaft/hole to make sure that the mobs are dead. This trap also works on minecraft pocket edition.
