“Players can enter the Mines on alternate Sundays (your kingdom successfully occupied/taken the veins. If not successfully occupied, you can use the vein points to exchange for Mineral Vein Gather Card at the Sacred Portal and use the Vein Gather Card to enter a vein not in your own kingdom) to collect.
Once inside the vein, the player’s castle will automatically enter the safe zone of the vein, where only the mines within the safe zone can be collected. The safe zone basically consists of level 1 mines, but if you want to collect higher level mines, you need to use the advanced teleport to enter the non-safe area.
Castles in the non-safe area can be attacked and mines can be plundered. It is recommended that players take care of their own safety when collecting mines and use war guards when collecting.
Castles can be attacked regardless of whether they are in a safe area or not.”
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Game Guide for Clash of Kings