In Atlantica Online at level 80-90, the Shogun Castle of Death will be by far the most grinding you will need to do. There is a 10 level gap between this dungeon and the next and the dungeon here will only get you to about 86 so you will need to grind heavily to get to the next dungeon. There are 2 sets of mobs on each floor. You will need to be careful because 1 group is much more powerful and is meant for the Cannoneer quest so they act like shadow dungeon enemies would.
The Ashigaru mobs on the first floor are fairly easy, but you need to watch out because they are Spearmen and Gunners. That means if a battle lasts for more than 2 turns, expect to be stunned. Otherwise they're easy. The second floor has Ninjas. They are trouble and use a powerful poison. Avoid them if possible. The third floor have the various Dancer and Samurai mobs. Also, you can buy atlantica gold to help you playing this game.
The Dancers are weak and easy. There are 3 types of dancers but they're all easy. Its nice to farm them since they have good drops for crafters. The Samurai mobs are a bit stronger but just as easy. The Samurai spam Dark Seed and the Kagemusha can spam Hex of Darkness. If you're not careful you can get Hell Fire used against you. Otherwise they're easy. Chances are you'll be farming here until you can get to the next dungeon.
You will need to farm the Kagemusha type for the Black Keep Key. Without it you can not face the boss of this dungeon. Nobunaga Oda is a very tough boss. He attacks all targets in a column and his attacks are powerful. His area skill is also one of the strongest. Its recommended that you don't face him without either scrolls or a fully upgraded and equipped level(Atlantica powerleveling) 80 team.
