The Empire's Land Tax

The land tax was the most important source of the empire's money. The departments of the prefect would have staff responsible for each diocese and each province. It may not be the most sexy answer to the question of Roman success or Byzantine survival, but this field by field assessment of tax was a huge reason why the empire lasted as long as it did. This income maintained an imperial bureaucracy who in turn maintained the armies and fleets which made the empire possible.