Chapter 15: How did they know someone had been in the tower?

According to Tom:

“Fresh oil in the lock. Mud on the steps — and these steps must be scoured clean by a real driving storm. Horses have been here within the last day or two. They were tethered to that iron ring. See the hoof marks ? See how the rust’s been rubbed off the ring? They had their nose-bags, too — look, there’s a little corn the birds haven’t found yet. I should think there were men here the day before yesterday.”