Simple Rule:If the Heroes win the Surprise roll, the Hero who opens the door may choose to move into the room after opening the door, before the monsters are revealed. If so, each Hero inside the room may reroll one failed hit roll during the first Hero Phase of the Combat turn.
Older Rule:Before opening a door, the leader may announce "Storm the Room". The Hero who opens the door may continue to move into the room after opening the door. Other Heroes and Henchmen may now complete their remaining moves. Then roll for surprise, adding an additional +1 to the Hero's roll.
If the Heroes win the surprise roll, the Leader places the Monsters anywhere in the room, monster Death Zones may not overlap. If the GM wins, the GM places the monsters anywhere in the room. Combat begins.
If "Open Window" placed monsters in the room before "Storm the Room", remove the monsters just before the door is opened, and replace them as above. The +3 DRM replaces the +1 above.
Comment: No overlapping Death Zones means you can't put 4 monsters in a square to fireball them, but you can place 2, assuming a 2x2 fireball. In a Lair, 4 Heroes should be able to kill a Champion and a Warrior in the first turn. A Quest room will be more dangerous, you might kill the Warlord, and get attacked by the remaining Champion and Warrior. If the Heroes lose the Surprise roll, they are likely to lose the Wizard if he isn't well protected.
Another option is to replace Champions in Lairs with a Jezzailachis team 50% of the time. The GM can shoot thru the doorway from across the room, forcing the Heroes to charge in, run away, or use the archer to shoot it out.