You can enter Siren’s Tower during Chapter 1 once the Grandport ship journey is blocked by rough seas. It stands north/northeast of Grandport Harbor in the Kingdom of Grandshelt; reaching its gate early does not let you inside.
Final Fantasy Resonance: How to Access Siren’s Tower
Accessing Siren’s Tower
You cannot open the gated door of Siren’s Tower before Chapter 1, as Rain and the crew will be sent here during this part of the story. To access the tower, you need to follow the story!
Finding the Siren
The main quest involves looking for a ship in Grandport to take you to Dirnado. Talk to the sailor at the docks; he will say the waves are too rough for any ship to sail. You need to find the Siren to calm the waves before you can head to the tower.
Entering the Tower
Once you’ve found the Siren, Fina can communicate with her and let the group enter Siren’s Tower. Although the door is still locked, this will grant you access.
Defeating the Siren Esper
After entering the tower, you must reach the top and defeat the Siren Esper in battle. Once defeated, you can summon the Siren during battles to aid you.