The Dark Lord's Tower is accessible directly after the introduction. However, your party probably won't be able to defeat the Guardians and the Dark Lord himself until you have become more powerful.

If anyone is struggling with the Dark Lord's Tower. Here are a few tips you may find useful.

Stock up on Ethers
I recommend bringing plenty of ethers with you before you enter the Dark Lord's Tower. The Dark Lord has a few Guardians you must defeat before you face him. The Dark Lord can also use "Mega Osmose", which drains MP from your whole party.

Summons
There are three summons in addition to the two Henriet starts out with. They are hidden away in the various dungeons and once defeated in battle, Henriet will gain the ability to summon them.

Neptune - Ice damage against all enemies and Blink on the party.
Phoenix - Fire damage against all enemies and revive fallen allies.
Bahamut - Massive none-elemental damage against all enemies.

Blue Magic
Cernus gains the !Blue command from mastering the Thief Orb and can learn some very useful spells. The two most useful for the Dark Lord's Tower are Mighty Guard and White Wind. Mighty Guard raises the party's physical and magical resistance and White Wind restores HP to the whole party. To get Mighty Guard, return to the Sunken Cave and find a Kracken. The Sorcerer's in Charton have White Wind. If Cernus has !Blue he will automatically learn these spells when he sees them cast.

Indomitable
This is a very useful skill that Tybalt learns by mastering the Thief Orb. This ability will DRAIN Endurance from all enemies, making Tybalt near impervious to physical attacks for the rest of the encounter whilst simultaneously making the opponents more susceptible.

Side Quests and Class Orbs
There are three quest givers in the game that can be found in the various towns in Bieune. Completing their quest will reward the party with a new Class Orb, which they can learn advanced abilities from.