Tower tactics.

In my opinion, the Tower is the weakest Castle. Heroes like Alamar or Gera, even with a specialization in magic, won't save the Tower! Solmyr is cool, but only at 200% difficulty or on small maps. The Tower is the most magical city, but in Heroes 3, there's a balance issue: warrior heroes are stronger than magic heroes because in a battle against a human, no one will give you the opportunity to cast spells like explosion, meteor shower, chain lightning, etc. Since warriors are stronger, they will have higher attack and defense stats + warrior skills + tactics. As a result, any skilled opponent will use tactics to move their troops and archers as close as possible, use Haste, and start dealing damage! In a couple of turns, they will turn your army into dust. The mage will have to choose between explosion or Haste? Maybe Slow, Dispel, or a Curse and Blessing, or Healing (the main ones are Slow, Haste, Dispel, and attacking magic). If they choose attacking magic, they need to defeat everyone with magic in 3-4 turns, which is doubtful. I'm not even talking about magic resistance and artifacts against magic, and maybe unicorns will be summoning your units, manticores will be paralyzing them, etc.

The Tower's only hope is to stay in the Castle and bombard enemies with ranged attacks and magic. As an old Heroes saying goes: ONE TOWER IN THE FIELD IS NO WARRIOR!

Added 10 minutes later
If I play against the computer, I set up 7 computer players in an alliance against me on 160% difficulty, XL+Underground, and it's nothing, I win, it's not difficult if you play as Shakti or Lora (Dungeon is my favorite Castle), you can also win with other Castles, but I'm very used to Dungeon.