Tactics for Rampart.

1. Specialization in Defense is not only present in the Swamp, but also, for example, in Mephala in the Rampart. And the Rampart is better than the Swamp again because of its magic, and the creatures there are stronger.

What is the point of competing with the Swamp in defense? The Swamp faction will still have higher defense because it is "in their blood."
And I personally have rarely seen Mephala as the main hero for the Rampart, and I wouldn't play her against the Fortress myself.

I would say that the strategy for the Rampart is quite standard and doesn't raise any questions: the main focus is on Elves, protected by Centaurs, Pegasuses, and Unicorns. That is, a focus on speed and maneuverability both during battle and during movement on the map. Or do you always carry Dendroids and Gnomes with you?

And, in my opinion, the Swamp faction has very good chances of defeating Elves in an open field. Dragonflies easily block the only ranged attackers, and "eliminating" these pests can be extremely difficult, especially if they have good defense. Wyverns cause a lot of trouble due to their speed, decent damage, and defense (by the way, it would be interesting to calculate how much damage needs to be dealt to them to kill one creature - it's quite a lot, and that's already clear).
Morgaroth Gorgons are a good counter to Unicorns and even Dragons, if they are summoned and present in the army. And, of course, Basilisks with their very unpleasant ability to turn opponents to stone, and Lizards, which are not snipers, but hold up well due to their good defense and health parameters.

2. The problem is that you need to capture a Castle first. When playing against the AI, this may not be a big problem (although anything can happen), but when playing against people, it is quite difficult to do without good magic, which the Swamp Castle does not have as much as the Stronghold.

I don't know what you mean exactly, but in most games, the second Castle is taken in the first or second week. Or do you manage to build the 5th level of the Guild in that time? :)