All castles in Heroes 3.

Well, if we're talking about the speed of building units, I think it's easiest to build monsters. In the Castle and Dungeon, you need to build all the warriors in order, plus the guild of mages. And that costs money and resources. But capturing a Fortress or Stronghold with the Grail is more difficult; their defense and attack increase significantly, plus they have real trenches. And in the Castle, you can either pass through once or use an exp spell to remove the enchantment, and that's it.