Oblivion gates, as you may see, named copied from the game Oblivion.
These huge portals lead you to the nether realm, they should only be accessed if you are doing the respective quest for the gate:
East Oblivion gate
Another one (south one) behind the Chuch district, at Skidrow .
One gate at the middle on the The Kingdom. (center one)
And last gate, at north of Snowville , at the Rumari Lake. (north one)
As you may already noticed, design was also copied from the Oblivion one.
Gates usually have spawners with deadly and dangerous creatures near them (Ghasts, succubus , zombie pigmen, golems and other hostile enemies)
Quests are really hard, they are like the battle towers, except towers are made of obsidean, are trickier, have better rewards and are harder.
To close an Oblivion Gate you must take the sigil stone, located at the top of each respective tower for the gate. (the sigil stone is a slime ball). In the nether towers , the higher you go, the harder things will become...