Armies composed of stronghold-based units have a balanced mix of ranged and hand-to-hand attackers. With the inclusion of the stronger units, these armies are particularly well-equipped to deal with attacks on other towns.
Pros
Stronghold is the cheapest town to fully build up.
Hero classes. Battle Mage is one of the most balanced classes in the game with a host of useful specials. Barbarians are the best fighter class available, expect attack ratings to rise fast and keep on rising. With great specials like Offense, a high-level Barbarian is a tough opponent.
Escape Tunnel: it is great to be able to retreat from a siege, so that you don't lose your best hero this way.
Cyclopes and Cyclops Kings can attack castle walls with their ranged attack. This is a vastly underestimated special that can tip the scale in an attacking force´s favor very quickly, especially when combined with a fast initiative-giving unit and Haste (Expert level for best effect). High Morale (to enable multiple shots) is also extra useful for them (the spell for this is Mirth) and if you have the room in your army, every additional stack gives additional shots at the wall! A stack of 1 Cyclops King 'sieges' as well as a stack of 1,000 (and twice as well as a stack of Cyclops).
Ancient Behemoth can win a 1vs1 against most 7'th level creatures.
Crag Hack is one of the best Heroes in the game, at later stages, he will deal incredible amounts of damage thanks to his Offense specialty.
Least expensive town as far as weekly troop cost is concerned for gold, though the Rampart town will often have a better economy in general by making use of the Treasury.
Possible to get Behemoths on third day, provided sufficient resources.
Cons
Stronghold creatures have the lowest defense rating of all towns.
Cyclops and Behemoth lairs both require a lot of crystal to build. This competition makes it hard to have them both out in the earlier times - you have to choose between one of them, while both are very useful.