Queen – “The Trap” [Campaign 1, Mission 4]

Guys, please help me out!!! Sorry for my noob question, I'm actually a pro at King's Bounty, but I decided to take a break and play Heroes, and I ran into a problem in the 4th mission "The Trap" – I can't get across to the other side (across the river). I found the Boots of Levitation in the Dungeon, but I can't figure out how to use them to fly across the river (((
Please help a noob!