From the introduction:
Gehennum is a volcanic and mountainous archipelago on a watery fantasy world. The islands are ringed with coral and clad in jungle, except where the people have cleared them to grow rice. The flora and fauna are generally like those of south-east Asia, though there are some introduced species (especially crops) and fantasy bring-ins.
The people of Gehennum are small, lithe, and brown-skinned. They have a sophisticated culture and technology, at least as advanced in every respect as the classical Mediterranean (Greeks and Romans), and in some areas (such as optics and chemistry) considerably better.
One important respect in which the Gehennese suffer by comparison with default fantasy cultures is that (there being no extensive grasslands on their world) there are no horses or equivalent riding animals. The very wealthy ride elephants, but on the whole heavily-armoured burgher infantry (not aristocratic cavalry) dominate Gehennese battlefields, politics, and society.
Another, less important, is that metals are relatively scarce. The Gehennese (and other peoples of the World of Isles) use glass, obsidian, bone, shell, and flint for cheap cutting edges. Where they use steel and bronze they use it with economy and the highest craftsmanship.