I've built a mob spawning trap but mobs are not spawning.
It's a tower that is 10 blocks long x 5 blocks deep x 35 blocks high. At the top of the tower (33 blocks) is a spawn floor. The spawn floor is 10 blocks long x 3 blocks wide. There are signs on the edges of the spawn floor.
I've used a mapper to find all the caves in the immediate area and they are all lit.
When I'm standing on the ground (35 blks below the spawn floor) nothing will spawn.
The game is set on normal, single player, and mobs will at night on the ground, just never in my trap.
Here is a pic of spawn floor (torch added for pic, it is completely dark normally)
Here is a pic of the whole structure (tower is built in the cliff).
I'm not sure what I am doing wrong. Please help.
Best Answer
There are two points to remember that may seem contradictory: first, like Nicholas pointed out, you need to makes sure that your mobspawner is the only nonlit space in a pretty big area around you (I think the area's actually about 150x150 blocks), but second, you need a lot of empty space around. In other words, minecraft will choose places that have a lot of surface area (not volume) to spawn mobs in, so if you were to have a lot of open caves beneath your mobspawner, the game will flag that area as more likely to spawn mobs in. You just also have to make sure that it's all well-lit, then you've effectively tricked the game into flagging that area to spawn mobs in, but then conveniently your mobspawner is the only dark area where it can do so.
I would recommend you take a look at this tutorial. You may not want that particular style of mobspawner, but it goes over the prerequisites of any good mobspawner quite nicely.