I noticed that swiping down doesn't do anything at the moment so why not make it swiping down "Undo" last action
So if last thing was you swiping up to try and combine tower/potion/item, swiping down undo that and gives you back that tower/potion/item.
If you accidentally replaced/built a tower in the wrong spot, just open up build menu swipe down and that last tower is back to what it was. (3 second limit from last action on this, to decrease abuse)
If you accidentally drank a potion, swipe down and you get the potion back. (also 3 seconds limit)
Of course it's limited so you can't just "undo" something if you combined it into something you don't want.
