Circumstantial evidence is a type of indirect evidence that is defined as relating
"... to a series of facts other than the particular fact sought to be proved... (that) is so closely associated... that the fact to be proved may be inferred simply from the existence of the circumstantial evidence."
The direct evidence concerning the charges against Tom Robinson are the bruises on her face. There is no other direct evidence--aside from Bob Ewell's statement that he saw the crime being committed--such as a rape kit testing, Mayella's blood on Tom's clothing, video footage, or other first-hand observers to the crime who are not related to Mayella. Circumstantial evidence relating to the trial would include Tom's admission that he was on the property and inside the house. It is obvious that Tom might have never been charged under 21st century standards, and a conviction would have been totally unlikely. As Atticus said all along, it all boiled down to Tom's word against the word of the Ewells.