On this level 23, you have to use the "control button" to switch between the two grannies faster. I kept on running out of time when I was using the mouse to switch between the two grannies, until I checked the help section about the different controls. Once you use the control button you can switch much faster and will have enough time to make it across the bridges with the key and get to the locked gate down below to open the gate with the key b4 the time runs out.