In CoreImprovements.xml you'll find stuff like

    <ImprovementType InternalName="CityHub1">
        <DisplayName>Village</DisplayName>
        <IsCityHub>1</IsCityHub>
        <RequiresCity>0</RequiresCity>
        <BarredTerrain>River</BarredTerrain>
        <BarredTerrain>SwampTerrain</BarredTerrain>
        <PreferredTerrain>Category:Land</PreferredTerrain>
        <PreferredTerrain>Forest</PreferredTerrain>
        <ZOCMinRadius>1</ZOCMinRadius>
        <ZOCMaxRadius>1</ZOCMaxRadius>
        <ZOCRadiusTurnsBeforeUpdate>0</ZOCRadiusTurnsBeforeUpdate>
        <BuildRadius>1</BuildRadius>
        <BuildableTiles>50</BuildableTiles>
        <CitywideDuplicateImpLimit>1</CitywideDuplicateImpLimit>
        <HideInHiergamenon>1</HideInHiergamenon>
        <LaborToBuild>0</LaborToBuild>
        <GameModifier>
            <ModType>LevelBarrier</ModType>
            <Attribute>Population</Attribute>
            <Value>50</Value>
            <CityUpgrade>CityHub2</CityUpgrade>
        </GameModifier>
        <GameModifier>
            <ModType>CityDefenseSummon</ModType>
            <Attribute>SummonUnitByClass</Attribute>
            <Value>2</Value>
            <Provides>+2 City Defender Party's</Provides>
            <UnitClass>CityDefender_Party</UnitClass>
        </GameModifier>
        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>Research</Attribute>
            <Value>1</Value>
            <PerTurn>1</PerTurn>
            <Provides>+1 Research per Season</Provides>
        </GameModifier>
        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>Gold</Attribute>
            <Value>2</Value>
            <PerTurn>1</PerTurn>
            <Provides>+2 Gildar per Season</Provides>
        </GameModifier>
        <AIName>Outpost</AIName>
        <AIData AIPersonality="AI_General">
            <AIPriority>5</AIPriority>
        </AIData>
        <UnlocksShopItem>SaltedPork</UnlocksShopItem>
        <UnlocksShopItem>Club</UnlocksShopItem>
        <UnlocksShopItem>Staff</UnlocksShopItem>
        <ArtDef>Art_CityHub1</ArtDef>
    </ImprovementType>

However the game IS hardcoded to use population for levelling up, that cannot be changed. But the values are there at least.