Can the center 'Arena' region be turned into a guild's Homeland? 

[Answer]: No. The center region can not be turned into a guild's Homeland. 

Will there be Gem tile after the event?
[Answer]: Yes, the #1 ranked Guild in the arena will win the Gem Tiles for their home Kingdom!

Will there be any medals for this Arena?
[Answer]: Yes, there will be Guild War Medals for this round of Arenas.

Will players be able to change guilds on that Arena or join guilds from other kingdoms?

[Answer]: Players will be able to join another guild from their own kingdom in the Arena. It is currently NOT supported to join a guild from a different kingdom in the Guild War.

Will players teleport to the Arena individually? If a guild leader teleports to the Arena, it doesn't mean that all his/her guild will follow automatically, right? So it could happen that a guild leader is relaxing in the home kingdom while his/her guild members are fighting for him/her in the Arena? :)
[Answer]: Yes, players can teleport to the Arena individually. When the leader teleports into Arena, their members will not be automatically teleported into Arena and they can teleport at a later time when they are ready.

What if there is a tie in scoring Guild's territory points?
[Answer]: If 2 or more guilds score the same territory points, the Guild who owns the Center Arena Region wins. If it is still a tie, Guild who owns the most layer 1 Region wins. So on and so force.

Why does it cost Gems to mark and remove conquer targets? 

[Answer]: The feature to mark conquer targets is a great way for guild leaders to coordinate attacks among guild members. It comes with strategical planning when deciding the next target. To avoid people accidentally adding or removing a target, it costs some gems to do so. Therefore, please plan ahead and mark your targets strategically.