Tactics for the Swamp (Fortress).

to invisible
Generally speaking, that's all correct, but there are too many ifs in your post.

If there are lots of beehives, if you find sulfur (by the way, + if there's plenty of wood, you can forget about development without it).

All of this is unfortunately not so easy to pull off.
And what if Rampart finds crystal deposits and manages to build golden dragons by the end of week 2?
There won't be a trace left of the Swamp.

By the way, taking a Dragon Utopia in week 2 is a utopia :-)
Even if it's the simplest one. (Well, in theory anything is possible, but in practice it's hardly feasible.)
If you get lucky by week 4, you might be able to manage it — earlier is unlikely.

Defending a castle against the Swamp is a pleasure — 1 weak shooter + 2 flying units, the Serpents are too weak even though they're fast.
You wipe out all the wyverns with your entire horde, and then cold-bloodedly shoot down the beasts behind the walls.

Of course, everything changes if the opponent has expert Water Magic with Teleport (as Qwerty said), but swamp dwellers are precisely known for their weak magic.

My personal opinion, as I wrote earlier, is that the priority of towns by strength on a random map at 160% difficulty is as follows:
1 - undoubtedly Necropolis
2 - Conflux, Rampart, Dungeon, and Swamp. The priority of these towns depends entirely on the layout.

Well, I don't know, I rarely play on random maps and I don't play at 160, only at 200.
And only on L and XL maps. There, the Swamp absolutely does NOT DOMINATE, and Necropolis, by the way, isn't exactly the strongest either (unless, of course, they found the Cloak of the Undead King).