In Monkeypile Mode, you must stack all of your towers on top of each other. You can place your first tower anywhere you want (except on the path), but all subsequent towers must be placed on top of it. So that you will still be able to access the upgrades for the towers, they will show up in a bar at the top (you can click on a tower's icon to open its upgrade menu). Once you have placed your first tower, you are unable to sell it; so you must choose your first tower position wisely.