This campaign is fully dedicated to Stronghold. There is only one town of another type, namely a Dungeon in the third scenario (Clan War), located in the underground layer.
Possibly the hardest scenario is the third, since there are four heroes which should be invincible.
Upgrade your heroes in the second scenario (Taming of the Wild). When a hero has already reached 20th level (highest possible in the second scenario), avoid using him/her in battles. Visit Altar of Sacrifice in order to get 20th level, if there's no opportunity to get it in a combat.
One of your heroes may get Diplomacy, because near your starting town in the third scenario there are many wandering Stronghold units.
Build Hall of Valhalla in every your Stronghold town (+1 Attack).
It's a very good idea to conquer the Dungeon in the third scenario, as you can get 4th and 5th level spells there.