According to the report by K9, it was an isolated planet, habitable, with atmosphere and gravity approximate closely to Earth normal. One day was equivalent to 23.3 Earth hours and one year to 350 Earth days.
K9 Mark II reported it had high technological prowess but the peoples were primitive. This was due to the Three Who Rule's ban on advanced technology. The scans were likely registering the Hydrax as the high technology. The inhabitants did have some hidden technology such as radio transmitters. (TV: State of Decay)
While it was thought the inhabitants were brought to the world by the King Vampire, humanoids existed on the world in a society of villages ruled by great houses. Once the King Vampire arrived, many of the royal families were tainted by the vampire curse. (PROSE: Blood Harvest)
The planet resembled a level 2 system. A village was the its most prominent feature. It was surrounded by forests. Further afield were caves used by the rebels. In the centre of the village was the Hydrax, which looked like a castle. Rock had built up around it so it appeared to be on a hill. Beneath it were caves containing the burial chamber of the King Vampire. (TV: State of Decay)
The King Vampire went underground beneath the Hydrax. Over time the ship was covered by earth and came to resemble a castle. A village grew around it. The villagers served the Three Who Rule, who were in turn subservient to the King. (TV: State of Decay)
Visitors from other vilages became infected by the vampires and returned home to infect the other residents. Most, if not all, of these outlying villages had a vampire in control. (PROSE: The Eight Doctors)
The Fourth Doctor, Romana II and Adric landed and were introduced to the Three Who Rule. Adric agreed to become one of them and Romana was taken as a sacrifice. Adric attempted to free her, but was himself taken a prisoner.
The Doctor made contact with Kalmar's rebels and they attacked the castle. The Time of Arising came and the King began to rise from his chamber. The Doctor used a scout ship to impale and kill him. Without their ruler, the Three also died. (TV: State of Decay)
Before he left, the Doctor, Romana and Adric attended a party with the rebels to celebrate their victory. Hurda convinced Romana to aid her sick child in another settlement. Romana was led into a trap and Zarn took her prisoner. The Eighth Doctor and the Fourth rescued her and Ivo killed Zarn. (PROSE: The Eight Doctors)