How does Height play into hostile mob spawns?

Question

The Minecraft Wiki Spawn page goes into great detail about how hostile mobs spawn. But it omits details about how height plays into common spawns. It DOES mention how slimes require the chunks they spawn in to be at a certain height level, but I'm more concerned with how the rest of the common hostile mobs spawn. (Namely Zombies, Creepers, Skeletons and Spiders)

To clarify a bit:

Mobs spawn naturally within a 15x15 chunk (240x240 block) area around the player.

Ok, well, thats simple enough, one chunk is a 16x16 block centered on the player, got it. But, thats a two dimensional measurement, and this is a VERY three dimensional game. Is height irrelavent in such spawning mathmatics? If so, wouldn't that mean that I could be at sea level, with a dark cavern at bedrock beneath me, and have some of the mobs I want to slay spawn all the way at bedrock rather than at the same height I am?

Or even more prevalent, (and the main reason behind the question reveals itself) I have a monster grinding tower that I've built. The tower currently has 6 floors with each floor boasting a 2 block high space for the mobs to spawn in. If you include an extra block for the actual 'floor' of each level of the tower, that makes 18 blocks high. Now if I'm near enough and far enough (as the reqs require for spawning) to and from from the tower for mobs to spawn inside of it, how (if at all) will the different high levels of the tower affect my spawn rates inside each floor?

Answer

Except for Slimes, height does not affect mob spawning. Chunks are 16x128x16, and are loaded all at once. It is impossible for only part of a chunk to be loaded. So this means yes, it's possible for a mob to spawn near bedrock even if you're on the surface.

Note that this is not true for mob spawners. A mob spawner will only spawn a new mob if a player is within 16 blocks of it (calculated spherically).

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