This clip from Ableton's Learn Live series covers Emit's Cursor Mode Wall feature, showing how to draw particle walls directly onto the spectrogram interface and use them to redirect granular particles as they play back.
Switching Cursor Mode from Emitter to Wall lets you click and drag to place walls anywhere in the field. You can resize them by dragging the corners and remove them by double-clicking, making it easy to experiment with different obstacle configurations before settling on a layout.