A Kaer is an underground, warded city. Dozens, if not hundreds, were built during the Fourth World, in preparation for the Scourge. They allowed lots of of Name-givers to survive when the Horrors ravaged the lands of Barsaive. The science of building kaers was researched by the Theran Empire and spread to nearby civilizations (usually in exchange for submission to the Empire).
Kaers were built underground, as Horrors have trouble passing through living Earth. They were also protected by powerful magical wards. Some kaers survived the centuries-long Scourge, many however have fallen to the Horrors (a guesstimate 50% rate of failure may be applicable).
Some kaers were breached by powerful Horrors (either physically or Astrally) from the outside, others were penetrated by Horrors before they were sealed; and their occupants subject to a most disturbing and horrifying demise. Some were destroyed when their ecosystems failed.