All castles in Heroes 3.

my gameplay strategy for the first week is as follows:
two heroes loot all unguarded resources (wood and ore mines) within their reach, using the fastest units.
second week:
then, one of them (the main hero) goes to the Castle and buys level 5 and 6 (preferably 7) units. Clears out resource locations and castles.
all of this requires good micromanagement.