A case in which there are no witnesses and all the evidence is circumstantial might be a burglary where there might be things like fingerprints, blood from a suspect getting cut from a broken window, shoe tracks, or tire tracks.
The Martin case has a subject who was shot, witness statements, a shooter and a statement from the shooter, blood and injuries to both the subject who was shot and to the shooter, tape recordings of the dispatcher and Zimmerman prior to the shooting, and observations of the police and medical personal at the scene. If the case goes to a jury, there is a lot of "direct" evidence here.
Direct evidence for some elements of the alleged crime, but no direct evidence of who escalated the conflict into the range of lethal threat, which goes to both the depraved mind element and the self-defense argument. The jury will be instructed that they cannot convict on murder 2 without each required element being proven beyond reasonable doubt, and I don’t think the state has what it needs to get there, whether directly, circumstantially, or otherwise, based on Gilbraith’s admission.
However, a hole in the evidence of this kind actually serves the race baiters well. GZ will probably walk, but the troublemakers will, due to the lack of direct evidence on escalation, be able to project their racial stereotyping onto that blank screen. Those who would use this kind of event for political gain could care less about the fate of one or two individuals and their families. They only care about how they can leverage this into advancing their legislative and social agendas. They probably troll for cases like this on purpose; the greater the perceived injustice the better. Other, similar cases would be uninteresting to them if direct evidence precluded their ability to invoke the spirit of racial hatred from the haze of ambiguity.
Actually, circumstantial means anything not eyewitness. No one saw the actual shooting (unless I missed that which is entirely possible). But, there is a lot of evidence to be sure. There were audio witnesses and witness after the fact, but no eyewitness of the shooting.
In fact, about 90% of all cases rely solely on circumstantial evidence.