This location is only accessible during certain plot points
The Galbadia D-District Prison is a very repetitive place, with 12 floors that all look identical. Each has two rooms, the contents of which change from floor to floor. Of special note are the orange Moombas, who will help you to varying degrees based on your actions. Be brave and just, and they will help you. This help can come in the form of creating shortcuts (by removing the barrier between the up and down stairs on a floor) or even giving you Items when you speak to them.
Initially you'll control Zell on floor 7. The party's Weapons are on floor 8, so head on up. (There is a save point on floor 6, but the cells are all locked so you can't collect any treasures yet.) You'll have to defeat a pair of Guards to claim the Weapons.
Once you've recovered the Weapons, Quistis and Selphie join Zell. Change up your junctions and prepare for a rematch with Biggs and Wedge. It's not a difficult fight.
Your next task is to head to floor 13 and rescue Squall. However, before you do so, you might want to head down to floor 1 and collect some nice Items, including Combat King 001. You won't have another chance to do so.
Your path up to floor 13 will be easier if you don't have Squall give in during the torture sequence. (As a general rule, you'll get more help from the Moombas if you're brave and true.) Afterwards, Zell will be chased by Guards, and will have to fight when they catch him.
Next, you'll meet up with your remaining party members. You'll control Squall, Rinoa, and one other member (other than Irvine) on floor 9. From here you have to head up. Once you near the top, you'll switch to the remaining party members on floor 8. Your goal is to reach floor 3. (You won't be able to explore floors 1–3.)
When you return to Squall, head up to the control room. Head outside and you'll be attacked by an Elite Soldier and two GIM52As. Take out the Elite Soldier first, since he uses support magic. During the cutscene after the fight, just hold right to survive.
Initially you'll control Zell on floor 7. The party's Weapons are on floor 8, so head on up. (There is a save point on floor 6, but the cells are all locked so you can't collect any treasures yet.) You'll have to defeat a pair of Guards to claim the Weapons.
Once you've recovered the Weapons, Quistis and Selphie join Zell. Change up your junctions and prepare for a rematch with Biggs and Wedge. It's not a difficult fight.
Your next task is to head to floor 13 and rescue Squall. However, before you do so, you might want to head down to floor 1 and collect some nice Items, including Combat King 001. You won't have another chance to do so.
Your path up to floor 13 will be easier if you don't have Squall give in during the torture sequence. (As a general rule, you'll get more help from the Moombas if you're brave and true.) Afterwards, Zell will be chased by Guards, and will have to fight when they catch him.
Next, you'll meet up with your remaining party members. You'll control Squall, Rinoa, and one other member (other than Irvine) on floor 9. From here you have to head up. Once you near the top, you'll switch to the remaining party members on floor 8. Your goal is to reach floor 3. (You won't be able to explore floors 1–3.)
When you return to Squall, head up to the control room. Head outside and you'll be attacked by an Elite Soldier and two GIM52As. Take out the Elite Soldier first, since he uses support magic. During the cutscene after the fight, just hold right to survive.