Both Uses of

medieval
in
The Lost Hero

Buildings crowded together inside high walls like a medieval town, way older than any place Leo had seen before.†
Chpt 18 *

All around the room stood life-size ice sculpture warriors—some in Greek armor, some medieval, some in modern camouflage—all frozen in various attack positions, swords raised, guns locked and loaded.†
Chpt 20

Definitions:

(1) (medieval)
relating to or belonging to the Middle Ages

(The period of European history beginning with the fall of the Roman Empire and followed by the Renaissance -- roughly 500-1500 AD)

Often thought of as a time of instability, superstition, plagues, feudal lords, and knighthood.
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much more rarely, medieval may be used to refer to something as old-fashioned and unenlightened.