If you want I can try and answer a few questions from a legal perspective. I'm not a historian but I did study law and had a few courses on Roman Law.

The Roman Law I studied is limited to the time of Justinian and the Corpus Iuris. The Corpus Iuris is, of course, the Roman Civil Law (civil law in the literal sense not as the classification of civil vs common law) but criminal law is (mainly) a part of Roman Civil Law as well.

The source I'm using for what I'm typing is based on a dutch textbook I have lying around from law school: Hallebeek, Jan, Fons et Origo Iuris

To start there are two general forms of legal action: those based on a right and those based on an infringement on a right to possession, the interdicta.

The difference between them: in first type of action, a judge will judge whether the claimant has a legal basis, if he has said right. In the second type a judge will only look whether the claimant has legal possession of the subject.

These actions can be started in two ways: the actiones directae and the actiones utiles. The difference between both is that for the actiones directae there is direct legal precedence in the sense that they are prescribed in the Edicts. If there wasn't a literal legal precedence, they had to use a actio utiles which I would classify as a "custom made" action because there was no prescribed legal precedence.

Now I arrive at the hearth of your question: the legal process. The legal process is made up of three parts: the Principium Litis, which is the phase in which the parties give their arguments and they frame the legal question (litis contestatio), the second phase is the Medium Litis. In this phase the parties further elaborate and bring forth proof. Finally, there is the Definitiva Sentetia, which is the final judgment of the judge.

Your question is mostly based on the first and second phase of the process.

In the first phase a defence lawyer would be able to bring forth a few procedural arguments: exceptio and praescriptio. These are purely procedural arguments in the sense of: the claimant is not competent, the judge is not competent, a contractual agreement between parties, the defendant was forced to do something (exceptio quod metus causa) or the claimant is unreasonable/not fair (exceptio doli mali).

In this phase we can already see the importance of the defence lawyer: a "normal" civilian usually isn't schooled in procedural law.

In the second phase the importance of a defence lawyer is also very clear. In this phase the claimant has to bring forth proof. The legal rule in this phase is: the one who claims something has to prove that claim. (there is a reversal of this rule in cases of a clear presumption in favor of the claimant.praesumptio).

Now, the most important forms of proof in these cases are: what the parties state and sworn statements of the parties, written proof (contracts etc) or testimonies from witnesses.

Testimonies of witnesses are only used if there are at least two witnesses (unus testis nullus testis).

Now, seeing as you're talking about criminal law with a defence attorney I'll take a look at the more criminal aspect of the Corpus Iuris: the iniuria. The Iniuria are both physical and verbal harm to a free person. A few of the examples are: the membrum ruptum or the abuse of handicapped persons, the os fractum or breaking bones, the adtemptata pudicitia or libel to name a few.

Based upon what I've written beforehand we could reasonably conclude that a defence attorney is such cases would be able to try and find an exceptio or could try and prove a lack of physical evidence: my client didn't hit that person or break his bones, see his arm isn't broken nor hurt! or call two witnesses that corroborate his clients side of the story etc etc.

I hope I've answered your question and I hope I didn't make any mistakes seeing as it's been a few years since I've really discussed Roman Law.

What a coincidence, I'm just learning about all this stuff in school!

To answer your question: even though hard physical evidence like DNA testing and fingerprints werd not yet invented, cases could still be won in favor of the defense. You see, roman lawyers, or oratores, were not really legal scholars as we know them today. They did contact legal advisors, but their skills were mostly public speaking: in a time were evidence was hard to find, you had to rely on your moving words and reasoning instead. In turns, the defense and the accuser had to reason soundly to try to convince a jury of judges called iudices, who would then vote wether the accused was guilty or innocent. Roman lawyers were as much writers as attorneys, many of them even published their most succesfull speeches as books.

This is greatly displayed in one of the speeches of the famous Roman Lawyer Cicero, 'pro Roscio Amerino', 'in the defense of Roscius Amerinus', accused of the murder of his own father. Though the accuser had a great motive with eikdence, and a few witnesses to boot, Cicero was able to denounce both witnesses as unreliable, through the principle of 'cui bono': who benefits (from the murder). By using written contracts, and other, more reliable witnesses, he was able to find the holes in the accuser's reasoning, and find the 'real' killer, with a better motive. Thus, Roscius was found innocent by the iudices.

However, Roman court cases were certainly not as fair as today. There is evidence of people 'selling themselves' as false witnesses, for example to lie about seeing someone kill someone else. The prosecution did have a very good reason to try to win, though: when an accusation was found false, the false accuser would have the letter K branded into their foreheads, which stands for 'kalumnator', or false accuser. That way, everyone they met could immediately identify them as dirty liars.

In classical Athens, at least, (which is the time and place in ancient Greece about which we know most, which is to say anything at all) one did not simply hire a defence lawyer, for the simple reason that there weren't any defence lawyers. Nor were there any prosecution lawyers. If you wanted to take someone to court, you had to get up and speak on your own behalf in front of the jury. You could get a friend to deliver it for you, as probably happens in Demosthenes' speech Against Leptines, and certainly happens in For Phormio.^1 You could also speak yourself, and then call a friend to speak in support of you as part of your speaking time, as happens in Against Neaira.^4

Finally, you could ask a speechwriter to compose a speech for you, though you would still have to deliver it. Officially, you weren't supposed to actually pay a speechwriter: you were meant to have a friend do it for you as a favour. We know that Antiphon, for instance, was accused of selling speeches.

The other misconception you are laboring under is that evidence is the only thing that counted in presenting a case. It isn't today (as much as modern legal systems try to make it about the evidence) and it sure as hell wasn't in classical Athens. Michael Gagarin has written a paper about this: essentially he argues that legal speeches in classical Athens attempted to tell stories which the jury would believe, rather than/in addition to relying on physical or testimonial evidence. The extreme example of this is in Antiphon's Against the Stepmother for Poisoning, in which there are no witnesses and no physical evidence whatsoever, and Antiphon essentially tries to bring his case via an extended mythological parallel with the story of Clytemnestra.

1. Probably Demosthenes wrote and delivered both of these speeches. We know that someone other than Phormio himself delived For Phormio, and Deinarkhos tells us that Demosthenes did on one occassion represent Phormio; presumably the two cases are the same.^2 It is probable that someone other than Chabrias, on whose behalf Against Leptines was given, delivered the speech, because we think that Chabrias was still a minor (and hence not allowed to speak in lawsuits) when the case was brought.^3

2. MacDowell 2009, Demosthenes the Orator, p.109

3. West 1995, "The Decrees of Demosthenes' 'Against Leptines'", p.239

4. The case against Neaira was brought by Theomnestos; almost the entirety of the speech against Neaira was given by his father-in-law Apollodoros (who almost certainly wrote the speech).^5 We're not really sure why Theomnestos brought the prosecution, rather than Apollodoros; MacDowell thinks that originally Apollodoros did intend to bring the prosecution himself.^6 IIRC, Christopher Carey suggests that Theomnestos brought the speech because in some cases in Athenian law, a failed prosecution was liable to penalty, and that Apollodoros, a prolific litigator, didn't want to risk being disenfranchised. IIRC, it's in Carey's 1992 edition of Against Neaira (probably the intro, but possibly in the notes), but I don't have access to a copy atm to check.

5. Kapparis 1999, *Against Neaira", p.29

6. MacDowell 2009, p.121