You step out of the woodlands that you've been exploring, into a small clearing that is mostly filled by a tall stone tower. It's clearly old, and it's uncertain exactly how it is standing, given the way it seems to defy the expectations one might have of a tower. Simple things like the standard in towers of stacking in a single column upwards, and remaining more or less stationary. Admittedly, many towers have been known to sway a little, given a strong enough wind, but this one seems to be... spinning? It's not fast, not here near the ground. It was barely noticeable at first, in fact. But when you look up, the only way to describe what's happening at the top of the tower is that the whole thing is whirling. As such, it's a little difficult to tell whether one is in fact looking at a tower at all, or whether it's a very slow moving tornado with an extremely sturdy construction of stones.
At the base of the tower is a large wooden door. The door seems to remain stationary, despite the rotation of the surrounding stones. As you move closer to it, you can see golden curly script hovering in front of the door itself. It says "Enter, Friend", then in slightly smaller letters "Or feel free to leave a note in the box". You noticed that there is a small mailbox to the right of the door.
What do you do?