What tactics should I use with Dungeon against Stronghold in online play?

Comment, please, and suggest a tactic for playing online on a Medium (M) map against the Citadel...

Are you playing as the "Pawn"? :) In that case, I would take Swamp instead of Dungeon – in my opinion, they complement each other quite well...
-Gnolls produce Goblins;
-Fast Rukh are balanced by fast Wyverns;
-With Wyverns and Hydras, you can defend against even Cyclops...
-Dragonflies can take down Orcs, etc.

This is just based on my observations...

As for Dungeon, it would be better to take the "cheating" hero, Dimera – a specialist in Starfall. If you manage to level him up and build a mana vortex, he will cast his mega-spell several times during the battle, which will deal huge damage...

In my opinion, in this matchup, it's generally better to focus on magic, in which the Barbarian faction is not so strong.