Actually, Castle is not the most expensive in terms of any of the resources...

Town----------Gold--Wood--Mercury--Ore--Sulfur--Crystal--Gem

Castle---------111000---147-----42-----122-----42----47----42
Rampart---------106000---127----30------142-----30----95----45
Tower---------111800---135----48------135-----43----49----89
Necropolis----97400----145----40------135-----50----45----65
Dungeon------112400----142----40------142-----82----32----45
Inferno------115300---120----86------125-----51----28----36
Fortress-------92400---185-----29------105-----49----10---10
Stronghold-------92200---157-----10------152----10----60----15
Conflux--101 800---137-----65------140-----38----33----35

The cost of a fully built town, including the Mage Guild, is shown.