I'll explain about altars.
There are four altars in the Tower.
The first one you'll find is on floor 4.
Next, on floor 12.
There is one altar in each area except for the third (Floors 21-30).
The higher you are in the tower, the more powerful the altars get.

The alter in the first area is too weak.
Therefore, you can clear this area with no alter.
You can save one blue key that way.
And you who have this key will be able to go by another way in second area.