Gates are portals that open to dungeons in different dimensions.
It is not certain when exactly the Gates started to appear.
Gates are opened by feeding on the negative emotions of magical creatures. Even Muggles are included in Magical Creatures because every creature on Earth has some magic, but not everyone can use magic.
If the Dungeons are not defeated within a certain period of time after they are opened, a Dungeon Break occurs and the Creatures or Monsters within the Dungeon are released. These Dungeon Breaks have occurred many times in the Old Ages due to the lack of Magic Users.
Therefore, Magic Users all over the World have continued to hide the existence of Gates from Muggles while cleaning these dungeons to protect themselves, because the main reason for the formation of Gates was the negative emotions of Muggles.
Although the Dungeons inside the Gates were quite dangerous, the rewards were more than enough to justify the dangers. Items that could not be found on Earth or were much stronger and more valuable than the materials found on Earth could be found in the Dungeons.
Ranks of Dungeons, according to the British Ministry of Magic :
E Rank : These Dungeons are generally used for training of Hogwarts students and have a Low danger level.
D Rank: These Dungeons are either handled by the Lower Level Guilds or used for the Training of Hogwarts Upper Class Students, and their danger levels are low-Average.
C Rank : These Dungeons are usually handled by Ministry, their danger levels are average.
B Rank : These Dungeons are usually handled by the Guilds of the Wealthy Pureblood Houses, Their danger level are Low-High.
A Rank : These Dungeons were once managed by the Guilds of the Wealthy Dark Pureblood Houses, but after their downfall, these Dungeons fell into the hands of the Ministry and the Light side, their Danger level are High.
S Rank : These Dungeons are very rare and their danger level is National.