minecraft-java-edition
I have a sweet ravine balcony, that I've lit up, and partially fenced off. It's not connected to anywhere that mobs could spawn yet, come night zombies are at my door creepers are lounging around and spiders are crawling around! I didn't invite them!
Is there any reason for these mobs to spawn here when it's well lit?
If you're willing to carry a door on you, the quickest shelter you can throw up is to just dig a 1x2 hole into the side of any hill and throw down a door to seal it off.
If you don't have a door I suggest the following:
▓▓▓ ▓▓▓
▓▒▓ top down view
▓▒▓
▓▒▓▓ ▓ 2 rocks
▓▒•▓ ▒ 1 rock
▓▓▓▓ • you
The key thing here is that by being off to the side you aren't in range of arrows, but you still maintain a path to light so you can see the sun up (you can also listen for music, but I often disable sound). You close off the entrance with single rocks so you leave a 1x1 path which zombies can't come down.
Update: According to Notch's blog post about changes coming in 1.6, the torches-on-fast-leaves bug is getting fixed:
This means that monsters can no longer spawn on leaves, either, as they are always considered transparent, even when using Fast graphics.
Monsters can spawn on any solid, non-transparent block which is at light level 7 or lower.
Signs, torches, rails, and the like are ignored when spawning monsters; they'll spawn on the block underneath just fine. This is why one-block-wide railways need to be lit.
Transparency means whether or not light passes through a block rather than whether you can see through it. Half-blocks, doors, stairs, and glass all count as transparent and prevent monster spawning. Leaves, oddly, count as transparent only when Fancy graphics are enabled. An easy way to test a block for transparency is to try placing a torch or sign on it — neither can be placed on transparent blocks. (Meaning that torches can be placed on leaves with Fast graphics, but not with Fancy.)
Torches put out light level 14, so monsters can't spawn within 5 blocks of a torch orthogonally or 3 blocks diagonally. If you put your torches in a diamond pattern, they can be up to 9 blocks apart, but it's best to move them closer than that on uneven terrain. (I use 5 myself.)
If you like, you can actually torch your treetops by changing your graphics to Fast, placing the torches, and switching back. Your torches will stay rooted, even though you wouldn't normally be able to place them that way. Personally, I think it makes them look rather cheerful. :-)
Best Answer
Mobs cannot spawn in a well lit area, but only a single square is enough to allow spawning. The light level must be 8 or higher to prevent spawning, meaning no cube can be more than 7 steps from a torch. You have to count vertical steps too, so a torch two squares off the floor can only be five horizontal steps from any floor square.
In addition, spiders can climb up walls, and thus are harder to keep out than other mobs. But the fact that you have creepers and zombies as well indicates that you have a lighting issue. Creeps can also fall down from heights farther than they would choose to walk; if the cliff above your balcony can spawn mobs, you may wish to light it or fence it to prevent mobs from pushing each other off the edge and falling to your balcony.
After you posted the screenshots, I made this graphic, illustrating some of the squares that have insufficient light. The green dot means that area is light level 8 or higher (within 7 cubes of a torch). A red dot indicates it is light level 7 or lower, and mobs can spawn there. As you can see, there is a large splotch of darkness on the right side of your balcony, where it is widest. Add a few more torches and everything should be fine. I did miscount a bit when creating the graphic: the leftmost two red dots should be green, as they are 7 cubes from the torch that is just out of sight on the bottom left. This leaves 5 squares visible in the image that are below light level 8.