The World of John Flanagan is one similar to medieval Europe with parts of northern Africa and Asia incorporated. Each country is listed, along with their real world counterparts and the reasoning behind each counterpart.
Nation Comparison Guide
Alpina
Switzerland /Austria
Name based on Latin name of Central European Alpine Region
Mountainous Terrain
Araluen
Medieval England
Land divided between aristocracy
Military - iconic knights and archers
Monarchy
Arrida
Fatimid Caliphate , possibly Umayyad Caliphate
Desert country
Same ethnic groups
Powerful military
Aslava
Russia /Kievan Rus
Slav is in the name Aslava
Slightly similar histories
Baralat
Unknown
Byzantos
Byzantine Empire Modern day Turkey/Bulgaria Captial: Byzantos is Istanbul
Celtica
Medieval Wales
Minor nation on island
Good relations with neighbors
Fairly weak army
Mining
Wales is part of the Celtic culture
Temujai/Eastern Steppes
Mongol Empire
Powerful horse archers/cavalry
Large empire
Similar tactics and army
Temüjin was Genghis Khan's birth name
Gallica
Medieval France
People speak French
Country split up between factions
Good wine and cooks
Helleno
Medieval Greece
Similar language
Names end in -os
Worship Ariadne, a figure of greek mythology.
Hibernia
Medieval Ireland
Customs similar to Medieval Ireland
Divided between six kingdoms
Hibernia is Latin for Ireland
Iberion
Medieval Spain and Portugal
Many sailors
Use hired assassins
Spanish names/language
Name similar to Iberian Peninsula, where Spain and Portugal is located
Indus
Medieval India
Similar naming scheme
Similar general location
Magyara
Medieval Hungary
Magyar is the Hungarian word for the Hungarian language
Middle Kingdoms
Imperial China
A traditional name that China has used to refer to itself, Zhongguo, translates as "Middle Kingdom" or "Central Kingdom"
Nihon-Ja
Samurai dominated Japan Possibly in the Kamakura Period
Customs similar to Medieval Japan
Feudal system same as Medieval Japan
Language based on Japanese
Nihon is the Japanese word for Japan
Picta
Celtic and medieval Scotland
Divided between tribal chiefdoms
No centralised government
Blue woad face paint and tattoos
Skandia
Early medieval Scandinavia
Similar culture
Raiding foreign coasts
Similar politics and titles (i.e. Jarl)
Similar religion
Sonderland
Medieval Greenland or Iceland
Large, northern islands
Similar terrain
Toscana
Roman Empire
Fighting style like the Romans
Ranking works similar
Export of olive oil
Italian/Latin language
Teutlandt
Medieval Germany
"Teutlandt" seems to be close to "Teuton", wich was the name of the people of the teutonic order
Ursali
Unknown, possibly Afghanistan
Santorillos
Santorini
Sovereign Island State
Previously unofficially governed by Myrgos
Located in the Constant Sea 's eastern reaches
Active Volcano, similar to what Santorini is considered
Santorini is considered a Caldera, just as described in The Caldera
Santorillos sounds like Santorini with a Greek "-illos" spelling.