Low-Tech Kingdom Population Density?

[Sunrunners_Fire]

Originally Posted by Brett

Would your constraints allow the people to practice a form of agriculture that is drastically more productive than European open-field seasonal agriculture in a cool temperate zone? With adequate water and labour, tropical rice agriculture produces several times more food per hectare per year, and I think there was a system in Mexico that was also far more productive than European practices: champa.

She didn't specify their food sources. I've been assuming an oceanic 'warm' temperate climate, with easy access to plentiful sea food. Rice should be acceptable. What is "champa"? (Google tells me about a Hindu kingdom and that it is a named location in Mexico, but nothing I can find about an agricultural method.)