Castrum Marinum was added in patch 5.4 of the Shadowbringers expansion.
Castrum Marinum becomes available at level 80 as part of the “Blood of Emerald” quest, which is given by Resistance Officer – The Lochs (X:11, Y:22).
The trial has a minimum ilevel of 485 to enter and rewards you with:
Allagan Tomestone of Poetics: 12