Greetings Secutor.
I may have the 1.1 version of this, but if you want to check it yourself look at the file descr_settlement_mechanics in the Data folder, bottom of the file.
Code:
<!-- city -->
<level name="village" base="500" upgrade="1500" min="400" max="2500"/>
<level name="town" base="1500" upgrade="5000" min="400" max="6000"/>
<level name="large_town" base="5000" upgrade="10000" min="400" max="12500"/>
<level name="city" base="10000" upgrade="20000" min="400" max="25000"/>
<level name="large_city" base="20000" upgrade="40000" min="400" max="50000"/>
<level name="huge_city" base="40000" min="400" max="100000"/>
<!-- castle -->
<level name="moot_and_bailey" base="500" upgrade="1000" min="400" max="1500"/>
<level name="wooden_castle" base="1000" upgrade="2500" min="400" max="4000"/>
<level name="castle" base="2500" upgrade="9000" min="400" max="12000"/>
<level name="fortress" base="9000" upgrade="18000" min="400" max="24000"/>
<level name="citadel" base="18000" upgrade="36000" min="400" max="48000"/>
So what I think it is is that the base represents how much you need. 1,000 for a wooden castle, 5000 for a large town, ect.