Aridius was a planet originally covered by oceans. Due to a stellar incident, its orbit around its suns turned the planet into a desert over a thousand years. The only remaining species were the amphibious Aridians and the octopus-like Mire Beasts. To control the Mire Beasts the Aridians blew up sections of the city that had been overrun.
The First Doctor visited Aridius, but was tracked by the Daleks, who pursued him in a time machine. The Daleks tried to force the Aridians to hand the Doctor and his companions over, but they escaped before the Aridians could do so. One of the Daleks was guarding the TARDIS, but was tricked into falling into the tunnels, enabling the TARDIS crew to escape. (TV: The Chase)
During the Last Great Time War, Romana II acknowledged Aridius as a planet which had been invaded by the Daleks. (AUDIO: Beyond)
Astronomical data
Aridius had high levels of oxygen and its gravity was a little greater than Earth's. (TV: The Chase)
Behind the scenes
According to non-narrative information in DWBIT 41, the Daleks later returned to Aridius, still remembering their encounter with the Doctor there. They wanted revenge on the Aridians and wiped them all out along with the Mire Beasts. Romana's mention of the Daleks invading Aridius in Beyond, in addition to planets exclusive to DWBIT Dalek Wars is evidently a reference to this event rather than the smaller-scale incursion seen in The Chase.