The Red Cities predate the giant web structure that houses the current provinces of Pocalo, but the people of the Red Cities were the ones who developed the technology to use cloud spider silk as a building material.
The earliest structures in the Red Cities were carved directly out of the mountainside rock. When the people needed to expand or repair the buildings, they initially used the cloud spider silk as a base for concrete or cement, mixing it with dust and gravel leftover from carving.
Later technological developments (domesticating and breeding the cloud spiders for different types and consistencies of silk) allowed them to build without the use of stone at all, thereby reducing the weight of the structures considerably.
The Valley is prone to earthquakes, so solid stone buildings are very susceptible to damage, whereas the flexibility of the web structures make them highly resistant to earthquakes.
The Red Cities were largely abandoned during the War of Light. Some have since been reclaimed (by bandit groups more often than not), while others were left to crumble. Although the Red Cities (both active and ruined) are technically governed by the laws of Pocalo, very few resources are actually spent enforcing those laws, giving the Red Cities a reputation as dangerous places to frequent.