This "gap" between Lingerie and the Furniture store is totally problematic. The green sign there indicates the car park (i.e. parking lot) Level 3 is to the right, just as the grille below on the Mall ground floor leads to level 2. But there's no grille on the top floor, to prevent intruders entering from Level 3 of the Car Park, and we never see anyone go that way until Season 3 when we see Bray heading for the roof that way.
In Season 3 also, when more rooms were needed, a sign appeared pointing left to "Mall Management." Here for the first time we find rooms with doors. This is where The Guardian installs himself, and where Tai-San as the new Supreme Mother will have her own room. There is also what I call the "Operations Room", a place May discovers and where she makes a hasty sketch of key locations in the City. Here are some links.
The Guardian's room Tai-San's room Operations Room
We also have a problem with the location of the Cage, supposedly to the left. It seems Cloud9 weren't quite sure where it should be. It is definitely in the Service Area, because that's where (for instance) Jack's water filtration system is, and the Cage is located nearby. In early episodes, people went to the Cage via this gap between Lingerie and the Furniture Store.
The two pics below illustrate this. Ebony escapes from the Cage in the night by luring Lex (who is supposed to be guarding her) inside, then locking him in. We then see her making her getaway via the "gap", looking in on Amber and Trudy as they sleep, before leaving the Mall via the sewers. Amber senses something and wakes; she finds Lex in the Cage instead of Ebony and raises the alarm. Here we see her returning from the Cage.
But later on in Season 1, when Cloe makes for the Cage to talk to Patsy, who is talking to Glen within, she goes via the stairway that hugs the Market lift.

This stairway is now the usual way that we see anyone (e.g. Jack, Lex) going to the Service Area.
Here we see Ebony in the Cage, held there after she followed Bray into the Mall and thereby ruined Zandra's wedding to Lex.
The Mallrats used the Cage to hold people, criminals we might say, pending trial, or until they decided what to do with them.
In Season 3 the Service Area cage is never used, and the Market Lift is used instead. It is called "The Cage" just like the original.
Below is a list of people who at one time or another were kept there.
Jack For Hoarding Food.
Ebony When she followed Bray inside the Mall. The Mallrats feared that she would bring the Locos in if they let her go
Glen Temporary measure only, to isolate him. He was soon moved to the Antiques shop where he died
Danni Season 2. Framed by Ebony, accused of trying to poison Tai-San
Luke, Salene & Tally Season 3. When the rebels take Brady from the Mall, the Guardian puts all three in chains. Luke because he's 2nd in command, and the two girls because they had most contact with Brady. He lets the girls go, and some Chosen guards loyal to Luke set him free.
The Guardian Pending trial, following the downfall of the Chosen. Ebony engineers his escape, and Ned's murder
May Season 5, for collaborating with Mega
Mega Season 5. after the rebels take the City
Another view of the Cage, and the Service Area.

Spot Jack's Filtration System nearby