...okay I'm a HOMM newbie...I've played a lot of turn-based games successfully, but I can't get a handle on this one...

I'm stuck on the very 1st custom game (Agrael's Trial) - I take over almost the entire upper level, have my buildings close to maxed out, and have 4 heroes with lots of troops...but, then about 6 Demon Lords appear from below and kick the **** outta me.

Not sure what I'm doing wrong...I've reset about 10 times and am still getting beat on "Easy" setting...this is really embarassing because I usually pick these games up really quickly - I'd appreciate any pointers or if anyone knows a walkthrough/hints list...I'm not into "cheat codes", but some sort of strategy hints would be nice.

I really like this game and am looking forward to going back to Heroes III and IV, but I need to get better at this 1st. Thanks in advance.