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.
Hobgoblins are excellent 1st level creatures due to their high speed. Giving them to secondary heroes they will give them a lot more movement points (only Sprites and Centaur Captains are faster). With 70-80 Hobgoblins, you can already take down 3 Behemoths and conquer a Behemoth Crag in the first week. Similar for other tough but slow monsters that are a single stack. This is possible due to their speed. Make 6 stacks of one Hobgoblin and the last stack has all remaining. This big stack is in the last slot (on the bottom when fighting). The strategy is as follows: The Behemoths are in reach of the Hobgoblins but the Hobgoblins are just outside the Behemoths reach. The big stack of Hobgoblins waits. One single Hobgoblin attacks the Behemoths to take the retaliation. Then, the Behemoths approach but can't attack because nothing is in range. After the Behemoths moved, the big stack attacks on the side where they have more space to run away. The round ends. In the next round, everyone runs out of reach in one direction, only one Hobgoblin stays behind in reach of the Behemoths to lure them away. Then, you get the same situation as before and repeat until the Behemoths are dead. You should get ~4 hits before they retaliated once against you. That is sufficient to take down the rest in melee 1v1.
Be aware that Tactics and Haste are incredibly important to get the rather slow Stronghold units engaged into combat.
Always have your Wolf Riders strike not as the first one so they won't receive the retaliation. They deal an immense amount of damage but are very fragile.
Expert Teleport and Earthquake are your spells when besieging a town with a castle (Teleport is hard to get though). Expert Teleport lets you teleport troops behind walls. You have only two mediocre damage-dealing shooters and one expensive flier. You need to get the Behemoths and/or Ogres involved. Consider splitting your Cyclops Kings into 2-4 stacks so you can take down the walls effectively. After 18 hits, the entire defensive structures are destroyed. With Expert Ballistics and two stacks of Cyclops Kings you can get 2 + 2*2 = 6 attacks onto walls per round this way. After 3 rounds, you have destroyed everything. In Round 2, some towers will already be destroyed. If you additionally have Expert Earthquake, you can take down the entire defensive structures in only two rounds. With four Cyclops King stacks and neither Ballistics nor Earthquake you will still destroy all defensive structures within two rounds.
Don't be fooled by the low AI value of Behemoths. Yes, they don't have a 2-hex breath attack, they can't resurrect dead allies and they are not immune to spells. However, they defeat all other level 7 units in 1v1 combat, except Haspids, and are also the cheapest level 7 units. Because they are not immune to spells, they *can* get resurrected. Their only downside is that they are slow - but that's what you can fix with Tactics and Haste. Their building has the fewest prerequisites of all level 7 buildings so you can start amassing them early. In late-game their defense-reducing ability scales very well and they deal absurd amounts of damage.
Beware of saving lives of your Thunderbirds. The more Thunderbirds is in a stack, the more additional damage will their lightning strike do. Cure them if necessary.
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 7th 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.
Freelancer's Guild may be helpful to get resources you need by getting rid of units which aren't very important (being weak, slow, and/or not from Stronghold).
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.
Castle
Rampart
Tower
Inferno
Necropolis
Dungeon
Stronghold
Fortress
Conflux 
Cove 
Factory 
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Goblin 
Hobgoblin 
Wolf Rider 
Wolf Raider 
Orc 
Orc
Orc Chieftain 
Orc Chieftain
Ogre 
Ogre Mage 
Roc 
Thunderbird 
Cyclops 
Cyclops King 
Behemoth 
Ancient Behemoth