The tower is the heart and soul of the game. This area is where you go to fight monsters and find your loot. The tower is divided into three seperate categories.
This is the first area you are given access to, and contains the lowest level monsters. In these floors, you will periodically find altars, which are unlocked with a skull that is found in the same area (possibly a different floor if in a multi-floor level). Activation of an altar will begin a fight sequence with a stronger monster than you normally encounter, but will reward you with a mythical key, and when used that key will give you a mythical fusing gem. We are currently working on compiling the experience per AP gains for these levels.
Heroic levels are unlocked later in the game, and are identical to the Adventure levels, but contain higher level monsters. Unfortunately, these levels will give pretty much the same loot as the Adventure levels, but with a few changes. Altars will not be present, but random monsters will drop Legendary keys that unlock necklaces and rings.
There are several types of special levels, including Daily, Limited Time, Once-only, and Rats.
Daily levels will cycle through on a daily basis, and typically contain five levels of difficulty. Rewards from these will be experience and (depending on difficulty level) fusing gems of various levels.
Limited Time levels are typically limited to one difficulty level, but will reward unique items that can only be obtained in those levels. The rotation time on these is typically a week or two.
Once-only levels is new to the game since update 1.1, and once completed, you cannot re-enter them. They will also have unique gear in them.
Rats levels appear periodically throughout the day (seemingly during the same time frames) and come in several difficulty levels, but only one per appearance. Completing these is typically fairly simple, result in far less experience, but always pay out dust items. They are a great way to stock up on dust for reinforcing items.