"Fortress towns are built at the edge of swamps and are often used as bases by the Beastmaster and Witch hero classes. Their armies are primarily made up of deadly creatures — born of the swamps, and then subdued and trained for warfare. While among them there is only one ranged attack creature, Fortress units are possessed of many special abilities to offset this deficiency." RoE manual
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While among them there is only one ranged-attack creature, Fortress units possess many special abilities to offset this deficiency.
Dragon Flies are extremely fast. They share 8th place by speed among town units with Efreet Sultans (not including 7 level creatures, these two are fastest in the game), being faster than all creatures in Tower and Stronghold - and it's on level 3! For early development, it rocks, since the hero may simply rush through the map having Dragon Flies only (even basic Serpent Flies are ok, as their speed 9 is decent); and in a big battle, they are a major problem since they remove positive spells casted on enemy units and cast Weakness, making them less effective.
Their swamp native terrain makes them move much faster than any hero (without pathfinding) in the swamp.
The Wyvern Nest is very cheap and has very low requirements compared to the other level 6 dwellings, allowing the Fortress town to get level 6 units in their army early even on impossible difficulty.
Bron is the only hero in the game to bring level 4 units - Basilisks - with him at the start of the game. (Except Illor in HotA.)
Cons
Fortress creatures have the lowest attack ratings of all towns.
The heavy focus on Defense can be disadvantageous in many cases, as a hero with high attack can kill their enemies before they have a chance to attack back. Attack can therefore be used as a type of defense, while defense cannot be used as offense.
The heavy focus on Defense combined with the fact that fortress only have got one shooter makes it more difficult for fortress to win battles without casualities than the other factions.
Some creatures have poor stats (in Shadow of Death some stats for Lizardman/Lizard Warrior and Serpent Fly/Dragon Fly were increased, but overall they are still relatively weak. In wake of gods, this can be fixed by use of some settings making Fortress creatures better).
Very slow 7 level creature, especially the basic version (with speed 5, Hydra is slower than six lvl 1 creatures, and there is no upgraded lvl 1 creature slower than Hydra; among lvl 2 melee units, only Dwarf, Walking Dead, and Zombie are slower).
With two fragile fliers (lvl 3 with 20 hp, lvl 6 with 70 hp) and one very weak shooter, the Fortress has a very hard time invading other towns properly. So Expert Ballistics, Expert Teleport and/or Expert Earthquake are a must.
Castle
Rampart
Tower
Inferno
Necropolis
Dungeon
Stronghold
Fortress
Conflux 
Cove 
Factory 
.gif)
Gnoll 
Gnoll Marauder 
Lizardman 
Lizard Warrior 
Serpent Fly 
Dragon Fly 
Basilisk 
Greater Basilisk 
Gorgon 
Mighty Gorgon 
Wyvern 
Wyvern Monarch 
Hydra 
Chaos Hydra