The strongest castle in Heroes of Might and Magic IV.

Phoenixes are definitely better in this regard because they tank and don't depend on external factors, such as running out of mana.

There's an interesting fact about phoenixes. Resurrecting angels, demons summoning devils, and nightmares' terror can be successfully countered with a magical leech and draining power (it seems Haven calls it that). Or with an imp or a witch's ring (but it takes 2 rounds). As a result, the aforementioned monsters lose the ability to cast spells (they don't have enough mana). But phoenixes cannot be countered in this way – after death, regardless of the amount of MP, they will still resurrect.