I agree, all castles are good in their own way, but let's consider building from scratch. There are no resources or gold. The most advantageous position in a Castle, in terms of level 1 units, is with improved gremlins (archers) – they are inexpensive and you don't need to engage in close combat.
Next, level 2. The best are archers (improved) – they shoot twice, and are also inexpensive and accessible.
Next, level 3. For me, Bellowerders are the best. I collect one of each resource + 1000 gold and set off. There is no penalty in close combat.
In conclusion, it is better to play small maps with those who have stronger heroes at the