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Game world is broken up into zones with each zone holding a quest hub and theme. Entrances to other zones branch off if the other but you're pushed "forward" to the next one in a fairly linear fashion. Each zone is open but has channels for population balancing with Blade And Soul Gold. People have been making comparisons to WoW but I'd almost say the Warhammer online zone structure might be more analogous.
If you need a comparison, bns' world is similar to wow's in several ways :
it is separated in different maps that you change to by going through a portal at set locations.
those maps are open worlds, you share monsters and ressource nodes with other players on that map.
dungeons are accessible at set locations in the world and also from the dungeon/arena hub, those are instanciated for your party only
On top of that, every map has several instances of itself (called channels) that you can freely swap between to avoid crowded spots or whatever you want to avoid. Note that those instances are not specific to your party, but people in different channels at a time will not see each other.
It's not a real open world, as the world would need to NOT be divided into multiple maps, but no mmorpg is a true open world. It's also not an instance based game like Guild Wars 1 or Dragon Nest.
The only ones would be like moving between different continents, but most of the actual zones in each continent are totally seamless. So WoW would be the closest thing to a true open world to Buy Blade And Soul Gold. Only dungeons and raids are not in the open world.
BnS is still not bad. I'm really impressed with the world, it feels big, just those mini instance portals between each map which is not that big of a deal since they take like 2 secs and you don't even get a loading screen. It might still psychologically affect your perception of the world though.