The map is large and great. There's plenty of room to explore. However, the constant attacks from large armies at the very beginning are tough.

I usually play on Recruit difficulty first. It feels like my Hero level was too low.

After defeating the first powerful Necromancer (Ibrahim, does that name ring a bell?), it became easier to defeat the remaining stacks.

What are the victory conditions? I thought I had to complete the missions, build a chapel, and defeat the Necromancers.

It turns out that defeating the Necromancers is enough.