A small addition to the castles question discussed above. Should'nt it really come down to the time period you are talking about? The main characteristics of the late Heian economy, for example, was a fleeting workforce not tied to the land and an extremely constrained work market. There were simply too few workers to go around, to both toil the land for those important agrarian taxes, and make large scale construction. The reason behind this was the decline in population in the previous period. For example, after the fire in 1180 (caused by the Heike), the Toodaiji shrine was reconstructed at a snails pace as there were not enough workers to go around. It took roughly 100 years to finish the reconstruction. And the Toodaiji was a symbol of the realm and had the highest priority. That, and the fact that warfare was largely waged on horseback and in open areas, severely curtailed the building of castles in the Heian period. However, as stated earlier above, castles were abundant in later periods and construction times shorter (the work market was different then).
