Are all of the Holy Grail War servants based on real heroes?

Question

Saber's character in Fate/stay night and Fate/zero is based on the real-life hero King Arthur. Are the other Holy Grail War servants also based on real-life heroes? And if they are, who are those heroes?

Answer 1

For the most part, yes, but there are some exceptions. I assume you haven't read the Fate/stay night visual novel, which answers your question. As such, spoilers for that below.

Some of the heroes are basically just reinterpretations of people known to have actually existed in the historical record. These are F/z's Rider (Alexander the Great), Caster (Gilles de Rais), Archer (Gilgamesh), Assassin (Hassan-i Sabbah); and F/sn's Assassin (Sasaki Kojirou; or at least, I think the consensus is that he was real) and True Assassin (Hassan-i Sabbah).

Other heroes are reinterpretations of fictional heroes, or of people of dubious historicity. In the world of Fate, these people are treated as having been actual historical figures who just happen to be known in the modern day through fictional writings. These include F/z's Saber (King Arthur), Lancer (Diarmuid Ua Duibhne), Berserker (Lancelot); and F/sn's Rider (Medusa), Saber (King Arthur again), Lancer (Cú Chulainn), Caster (Medea), and Berserker (Heracles)

And then you have the one huge exception, which is F/sn's Archer (that is, Rin's servant, not Gilgamesh). F/sn's Archer is actually Emiya Shirou, who is fictional.

There's also a delicate issue surrounding the various Hassans who are summoned as Assassins - the idea there is that all ordinary Assassins (not including weird exceptions like Sasaki Kojirou) correspond to a leader of the historical Assassin cult. The issue there is that there are basically no records (in the real world) of what the leaders of the cult were like, so their personalities and abilities in the world of Fate are entirely an invention of the authors (Urobuchi Gen and Nasu Kinoko for F/z and F/sn respectively).

Answer 2

The Tohsaka Family in the beginning provided the necessary land and called forth the Servants. The original design was to summon only heroes from history and legend. In the Nasuverse, there are a lot of our legends of heroes such as Hercules, King Arthur and Siegfried which actually did happen. However, their legends are changed in parts, such as King Arthur having actually been a woman who was made into a pseudo-male by Merlin (considering that Guinevere knew Arturia was a woman, Merlin's actions may have been on his own whims as a prankster).

Real life "heroes" such as Alexander the Great, Jeanne d'Arc and Gilles de Rais have their histories change to suit the fact that magic exists. Their Nobel Phantasms, the symbols of their heroic deeds, are also more magically based.

Servants such as Medea and Medusa should not have been able to be summoned as they aren't heroes. However, during the 3rd Holy Grail War, the Einzbern, having lost the first 2 wanted to win at all costs and created a new Servant Avenger who was Angra Mainyu. However, this Avenger class was very weak and was killed early in the war. However, Avenger was accepted into the grail and corrupted it. This allowed almost anything to be summoned, as the original rules of the Holy Grail had become distorted by Angra Mainyu's corruption. This may have happened before Saber was summoned during the third war as the Edelfelt sister's sorcery trait allowed 2 Sabers to be summoned, one aligned to good and one aligned to evil as in the fake Holy Grail War in Fate/Hollow Ataraxia. Arturia summoned with her Alter from as a second personality to duplicate the Edelfelt sister's Saber (the fake 5th war was based off of the 3rd war).

Originally, the Holy Grail required that those summoned under the Assassin class had to be one of the nineteen individuals known as Hassan-i Sabbah, the pseudonym for the leaders of a medial Islamic sect called Hashshashin. Hashshashin is the etymological root of the word "assassin". However, due to the corruption of the grail, Sasaki Kojirou could be summoned. Sasaki Kojirou does not actually exists in the Nasuverse at all. He was a nameless martial artist with some relation to the Ryuudouji Temple, and Medea used the temple grounds itself as catalyst to summon him. Of course, when Zouken got involved he was able to turn Sasaki into one of the Hassan-i Sabbah.

With King Arthur, King Arthur is a real hero. However, the commonly known Arthurian legend is mainly fiction based on Arthur's legend. However, in the Nasuverse, they did actually happen, i.e. Merlin, Morgan Le Fay, Arthur's Fate.

Now that's just the Fuyuki Holy Grail War. There are other Holy Grail Wars that occur outside Fuyuki and have different rules, like the case with the Moon Cell Holy Grail War. Because the Moon Cell have observed all of human history, any hero is summoned as their true self and not affected by how their legends/history has been perceived over time. This is why Nero Claudius Caesar Augustus Germanicus is summoned as a woman and looking a lot like Arturia because King Arthur was said to be a fusion between two individuals, one of whom was a Roman general. Vlad III is different from his Great Holy Grail War counterpart. The one summoned in the Moon Cell is more blood thirsty and mad in relation to who he was (in the Nasuverse), while the Great Holy Grail War version of him gives off a more of a vampire feel as due to how Dracula's legend was born from Vlad's actions.

The Great Holy Grail War occurs after the Greater Grail had been stolen after the 3rd Holy Grail War. It's not known if the 3rd war is the same as the one Avenger was summoned in or a different one in a parallel universe where he is not summoned. However, much like the Fuyuki Holy Grail Wars, the Great Holy Grail War summons heroes from both real history and from legends that occur in the Nasuverse such as Siegfried and Mordred. However, other Servants who wouldn't normally be classified as heroes are summoned such as Frankenstein's monster, Shakespeare and Jack the Ripper. Also, it seems Assassin doesn't have to be Hassan-i Sabbah.

On the this Type-Moon Wikia page, you can see a list of every known servant summoned in all the different wars, their know heroes and links to Wikipedia articles for most. Links in the Servant column go to that Servant's profile within the Nasuverse while the Identity Column normally goes to the Wikipedia articles if they have a known history/legend (one exception is of course EMYIA).