Walkthrough tips, Discussion of Campaigns [Heroes 5]

Has anyone completed mission 4, "The Trap," in the Haven campaign? Please help!
I think I've done everything: cleared two Dungeons and everything else,
built the Castle. What does it mean to encounter Alaron's troops,
and is it possible to cross the bridge there?

You need to enter the Seer's hut, receive the quest to bring the boots (they are in the Dungeon), and use them to fly to the other side of the river.

Ugh, I can't play as the Necromancers.

Guys, how do I complete the third mission? How many Castles do I need to develop, one or two, and how many heroes should I use, only Markalov or also the Queen? And where are the enemy Castles? I took the staff, but where is everything else?

I completed the entire Necromancer campaign, and I didn't want to complete the third mission myself because it's very long! I gathered a decent undead army with Markalov, captured the first enemy Castle, and then I encountered an aggressive Elven army... in general, this Elf destroys almost my entire army! Well, that annoyed me, and I didn't continue playing :-)
Regarding the location of the enemy Castles, you have a magic window near your Castle that shows you the location of the three enemy Castles. I didn't use Isabelle; she stayed in the Castle.