1. Hell is conical
Hell, as Dante described it, consists of nine concentric circles, going deeper each time as they get smaller, towards the centre of the Earth. Which of the nine you are condemned to depends on your sin, with circles devoted to gluttons, heretics and fraudsters. The centre point of the surface of the cone is Jerusalem. The river Acheron runs around hell, separating it from the outside world. Outside hell itself, but still part of the scheme, there are the people who somehow never did anything good or evil in their lifetime. Even they are punished for their neutrality, running round for eternity being stung by wasps while maggots drink their blood.
2. Hell is diverse
The modern cartoon image of Hell, with flames and pitchforks for everyone, is tragically bland compared with medieval depictions. This modern version is probably the legacy of Milton, who in Paradise Lost describes hell as "one great furnace" whose flames offer "no light, but rather darkness visible". Then again, he is setting it in the time of Adam and Eve when its only population is demons, so even his Hell might have livened up a bit later. In the medieval hell explored by Dante and painted by Hieronymus Bosch, punishments are as varied as sin itself, each one shaped to fit the sin punished. In Dante, sowers of discord are cut to pieces, those who take their own lives are condemned to live as mere trees, flatterers swim in a stream of excrement, and a traitor spends eternity having his head eaten by the man he betrayed. In Bosch, one man has a harp strung through his flesh while another is forced to marry a pig in a nun's wimple, and other people are excreted by monsters. This Hell is not a fixed penalty, but the fruition of bad choices made during our lives.
3. Hell is underground - maybe
In the Middle Ages, people generally thought of Hell as being underground, and there were legends of travellers seeing its smoke coming up through holes in the ground. Dante agreed, and because he assumed that the Earth was round, he had Satan at the bottom of Hell, with his waist the centre of Earth's gravity. Milton's Hell, however, is far from Earth. Paradise Lost is set in the time of Adam and Eve when the Earth was still perfect, so it would be incongruous for hell to be in the centre of it. Dante manages to do the whole tour of Hell, purgatory and heaven in less than a week. Meanwhile it takes Milton's Satan nine days just to fall from Heaven to Hell.