Also, while the show is entirely set in Latin America, and a Spanish dub exists, it was never broadcast, aside from some Latin American channels. Understandably, as Spanish conquistadores are the Big Bads.
The Spanish being the big bads is actually very debatable. If anything, the Olmecs are the Big Bads while everyone assumes the Spaniards are the Big Bads. There have been shown to be plenty of despicable people on both sides, with Malinche, probably being the worst you can get in this series, being an Aztec. (Even the Spanish Doctor who's traveling with her is disgusted when she suggests poisoning a village's river supply)
The whole show is very careful to reinforce the point that aside from the three children (and any other children they meet), very few are Black and White Morality "good guys". Aside from those mentioned above, at least one tribe of Native Americans tries to sacrifice the children, and even the fabled Heva civilization ended up in a war of mutually assured destruction with Atlantis. Arguably the central conceit in giving the medallions to children at birth is because they would only use them for the right reasons.
Evil Is Sexy: LaGuerra, Zares' villainous henchwoman, is quite the beauty and has attracted the attention of lot of male viewers.
Gateway Series: For UK, French and other viewers outside North America (outside of francophone Canada) this (along with Ulysses 31) was the Gateway Series to anime in the same way as Ranma ½ or Sailor Moon was for US viewers.
Growing the Beard: There were complains about season 2 regarding pacing, the plot, out of character moments, Idiot Ball, many Distress Ball and unfunny humor. The general census is that season 3 greatly improved and fixed many of those problems. The animation and the action also take a surprisingly turn for the better.
Ho Yayinvoked: A Puppy Love version with Esteban and Tao, who cling to each other when stressed or frightened, invade each others' space at any time for any reason, and generally behave like a married couple. Also, when Tao was forced to destroy the Solaris, only the sight of Esteban calling to him could cheer him. Also: Sancho and Pedro for the adult version, or Gaspar and Gomez for the villain version.
In the sequel series, Esteban is overjoyed to see his friends. He hold Zia's hands... and then he jumps into Tao's arms.
The third city of gold is collapsing because of an earthquake. Zia and Esteban try to get their medallions and don't return in time to get in the submarine. They actually made it out alive by riding an escape boat.
After rescuing Esteban's father, Mendoza, Pedroa and Sancho think of leaving on their own. Mendoza wants to become a sailor again and the duo wants to open a tavern in Barcelone. This doesn't stick and the trio is eventually back helping our heroes again.
In season 2, Zares manipulate our heroes in various Batman Gambits in order to facilitate the children discovering the City of Gold for him. In the end, all he needs is reaping all the rewards. But his greatest accomplishment is keeping his true identity a secret: he's truly Ambrosius, the diminutive eccentric and friendly alchemist our heroes have being working with near the beginning. By the end of season 2, there still is no one who has discovered his secret identity (save for Tian Li who was dumb enough not to warn our heroes.).
One-Scene Wonder: Father Rodriguez, Esteban's first Parental Substitute who raised him prior to the events of the series. While he dies in the first episode, his two scenes really show that he was a good and loving man. The gorgeous and sad music just after his death turns the death of this minor character into one of the biggest Tear Jerkers of the show.
Squick: Myeena is about the same age as the heroes, but she's engaged to a man who looks old enough to be her father.
If it helps, Myeena looks to be at least a couple of years older than Zia at least, and Wynacocha probably isn't THAT old- in his early to mid-20s would be more accurate than "old enough to be her father".
Ship Tease: Season 2 has a huge amount of Ship Tease, mainly with Esteban and Zia. You can see them constantly hugging and holding hands. When Zia saved Esteban's life, he gives her a smooch. Zia also has good deal of Ship Tease with Tao, Zhi, Prince Zhu and Gurban.
They Changed It, Now It Sucks: After the release of the preview trailer for the sequel series, which showed that the series would be animated with Cel Shading, a lot of comments pretty much declared that the whole thing was permanently ruined.
Very much a case of Rose-Tinted Nostalgia Glasses on the part of some fans. Speaking as someone who loved the original series, the truth is that what made it special was being more than the sum of its parts - while some of the artwork in the original series was breathtaking, the overall quality of the animation actually varied wildly from episode to episode.
The cel-shading of the first couple episodes has a very conspicuous computerized look, with robotic movements for the characters. But it gets much better as the show goes on.
Values Dissonance: Myeena, a tween, is engaged to Wynacocha, an adult man. Apart from Tao, who's disappointed she isn't available, everyone takes it in stride. Probably a case of Truth in Television; the concept of legal adulthood is a relatively new one and adolescents getting married was fairly common back then.
Hell, there's enough of it in The Bible!
Villain Decay: Gaspard makes his return in season 3. Rather than being The Brute, he's been reduced to a villainous Butt-Monkey. To illustrate this, the children weren't afraid of him anymore and they kicked his ass embarrassingly more than once.