Tactics for the Swamp (Fortress).

1. To move quickly, it's enough to deploy a few Gnolls with your starting hero, and with new heroes, deploy the entire army (if there's no need to fight) and take one Gnoll. There won't be a movement penalty. And the enemy will have a long way to crawl through the swamp.
2. The Castle's main ability is defense, but it helps archers much less than it helps melee units. The only archer is weak, but the Fortress can easily kill enemy archers (Tactics is often researched, and there are Serpents).
3. Three levels is, of course, a little, but this is compensated by good defense. Besides, what prevents you from taking another Castle and developing the Mage Guild in it?
4. Sieges are indeed difficult in the early stages of the game, but Dragonflies are very helpful: 1) The Fortress gets the first turn (at the beginning and in the middle of the game) in battle; 2) Dragonflies can easily block enemy archers.

It's hard to prove this in words. It needs to be shown in battle, but that's not up to me :(

1. To move quickly, it's enough to deploy a few Gnolls with your starting hero, and with new heroes, deploy the entire army (if there's no need to fight) and take one Gnoll. There won't be a movement penalty. And the enemy will have a long way to crawl through the swamp.---Yeah, as if Gnolls are the slowest units in this Castle... and you could say the same about all Castles... at the end of the turn, you can leave Angels with the Humans, upgraded Vampires with the (potentially slow) Necromancers, and they'll all move like lightning... but that's not really indicative of the game. Most of the time, you'll have to move slowly, and frequently splitting your forces at the end of the turn is also a risk. A Human hero might teleport or fly in from the darkness and first attack the weaker hero with the main forces, and then the main hero with the fastest unit. That would be very frustrating, and computer heroes often like to pop out of one-way portals when you least expect them.
2. The Castle's main ability is defense, but it helps archers much less than it helps melee units. The only archer is weak, but the Fortress can easily kill enemy archers (Tactics is often researched, and there are Serpents).---It's not convincing enough to play this Castle just because of its defense. Tactics is researched by most heroes of all Castles, so that doesn't surprise anyone. But Air Magic is a rare guest for the Swamp Castle, and mass Haste is something fantastic for them (at least, in all the games I've played against the computer and other players with this Castle, no one has used it; at best, they Haste one unit). And usually, it's quite easy to fight against them: you apply mass Slowdown at a certain stage of the battle. Due to the lack of good archers, the Swamp Castle has to move forward. Serpents usually only slow down an archer for one turn (or at best, two), and then they are quickly defeated, and the archer continues its work. Due to the lack of good magic in the Castle, you usually don't have to expect anything fantastic from them, like an explosion, in the early stages of the game.
In short, fighting against the Swamp Castle doesn't cause me any particular difficulties.
And to be honest, I wouldn't want to have them as neighbors, just like the Barbarians (you can't learn anything in their Castle anyway... and you'll have to fight :( ).
3. Three levels is, of course, a little, but this is compensated by good defense. Besides, what prevents you from taking another Castle and developing the Mage Guild in it?---Yes... it's one thing to learn it in your 5th-level Castle (with the same portal town and possibly teleport if you're lucky), and quite another to find, conquer another, and possibly build it there too... do you see the difference in time? And when playing only with humans, it's not that easy to capture someone else's Castle without good magic and archers.
I personally don't see anything good about this Castle.