Stuck in Chapter 9-Caverns

[7Sinner]

Hello

I'm hoping someone can help me out as I can't find a walkthru for the game. I've made it to the area in Chapter 9-Caverns, where there is a large winding staircase lit by torches. When you reach the top there is a door which i can't open and three oval windows, two of which have monsters behind them and one that is partially open. There doesn't appear to be any way to proceed from here. Any help would be greatly appreciated.

Thanks.