Porticus Olidus

Porticus Olidus is a hall on the seventh floor of Hogwarts Castle, around the corner from the Room of Requirement. It can be accessed through the Pungent Passage, and gives way to the Detention Escape Route.^[1]

Porticus Olidus was used, on occasion, by Dumbledore's Army members, to access the Detention Escape Route and evade Argus Filch after hours.^[2]^[1]

Behind the scenes

A "Porticus" is an entrance-porch of a church. "Olidus" means, in Latin, "evil-smelling", "fetid".
On the official Marauder's Map replica made by the Noble Collection (based on the map prop, as it looked like in the film adaptation of Harry Potter and the Prisoner of Azkaban), the Porticus Olidus seems to be located in the dungeons.^[3]

Hogwarts Castle
Locations on the seventh floor
Attic · Bean Challenge rooms · Classroom 7A · Classroom 7B · Classroom 7C · Dark Tower topmost cell · Disused bathroom · Divination Classroom · Fat Lady's Corridor · Filius Flitwick's office · Grumpy Staircase · Gryffindor Tower reading room · Gryffindor girls' staircase · Gryffindor Boys' Dormitory · Hall of Hexes · Knights' Room · North Wing · Porticus Olidus · Pungent Passage · Room of Requirement · Runic Corridor · Seventh Floor Corridor · Sybill Trelawney's office · Divination corridor · Staircase to the seventh-floor corridor
Portraits, statues and other artwork located on the seventh floor
A Young Professor McGonagall · Barnabas the Barmy · Bridget Wenlock · Sir Cadogan · Caged monkey · Edward Rabnott · Elephant · Elizabeth Burke · Fat Lady · Flowering almond trees · George von Rheticus · Giraffe · Godric Gryffindor · Healers · Hippopotamus · Journeying wizards · Lachlan the Lanky · Mother and baby · Oraclitus Spheer · Staring Portrait · Three young ladies · Tiger · Two lovers in a graveyard · Valeria Myriadd · Wailing Egyptians · Witch flying to the moon · Wizard in black flowing robes · Wizard in blue · Wizard with a broomstick · Wolfhound