From Medivia Online Wiki
Coldridge is an ice island occupied mostly by Orcs. It's located next to Forgula. Once you sailed here with NPC Sven (Northport - Exploring the North Quest) you are able to travel here whenever you want. It is recommended to bring a pick with you. The isle is small but it is a good place to hunt especially if you are doing Orc Bowman task (80 kills per hour) or Giant Spider task (in Spider's Nest). There is also a mailbox which you can use after completing the Postman Quest. This island's underground isn't a safe place for low leveled adventurers, because only death awaits there (read as Undead Dragon, which you might meet if you go too deep).
Keep in mind that you need to open the Orc Fortress gate through Orc Camp first. A Wooden Key 6050 provides access. You can find more information about that here. In order to go to Orc Camp you need to go north from the Sven's raft, there you can find a hole which allows you to go underneath the water.
On this small island you might meet monsters