Readings & Meanings:
floor (of a building), rank
The Edo Period was the period when the social hierarchy of samurai, farmers, artisans, and merchants was established.
I fell down the stairs and hurt myself.
The couple who reside in the third-floor apartment are always having domestic squabbles.
阝 on the left, so the radical is Kozato, an abbreviated form of 阜, a picture of a pile of earth.
阝, Kozato, is different from 阝 (Oozato), as explained in my post on 降.
The phonetic element is 皆. We should already know 皆 as meaning "everyone."
比 is on the top of 皆, and we should know it from 比べる, to compare. 比 is a pictograph of two people lined up.
The bottom part of 皆 is a squished version of 自, self. It acts as an action indicator.
Let's just think of 皆 in 階 as "line up" and 阜 as "higher earth." Thus "rank," as in when you line up people you will place the higher-ranked people higher than the rest. You can also think of "stairs," if you think of a long line of people, one after the other, marching up the stairs. If you've ever been in a busy train station in Japan, you can easily imagine this.