In general, each Castle in Heroes 3 is good in its own way, but I don't play well with every one of them. I prefer the Swamp Castle because of its clear ease of development. To build dwellings for creatures of levels 1, 2, 4, and 6, you don't need any resources other than wood, stone, and gold. This is an undeniable plus, especially on small maps. Furthermore, the Swamp territory itself often creates a significant problem for the enemy.
Add to this the "armored" nature of the Castle, the достаточно хорошие abilities of some creatures, the ability to quickly build Wyverns, and decent heroes – and you get an army that can not only put up a good defense but also become an aggressor.
P.S. Who disagrees?