Both Uses of

medieval
in
The Idiot

On a sheet of thick writing-paper the prince had written in medieval characters the legend: "The gentle Abbot Pafnute signed this."†
Chpt 1.3 *

I think the poet desired to embody in this one picture the whole spirit of medieval chivalry and the platonic love of a pure and high-souled knight.†
Chpt 2.6

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.