Question. If my hero has high speed during battle, then I should be the first to move in battle. If I am misunderstanding something, please explain.
Yes, a potion is a good thing, but I don’t remember it being sold in the Castle.

1. Potions with "Endurance" are sold in the Barbarian and sometimes Chaos Castles!
2. The first turn generally depends on three parameters: "Luck," "Speed," and "Morale"... if you have more points of these values than your enemy, according to their parameters, then you will definitely go first, even if you are attacked on the adventure map... That is:
"Let's say you have Speed=7, Luck=2, Morale=3......and the enemy has Speed=5, Luck=4, Morale=1, then you will definitely go first"
.....The correspondence of these parameters may look different, and it may also be possible to do without some of them...
P.S. (Use RandomMap, and by the way, your opponent can only deceive you if they join your game over the network and can change or delete your artifacts:) )...
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In general, browse through the section for Heroes 4 and read the topics... there you will find a lot of interesting things.......