A History of the Fortress
The People with Horns
The Fortress contains images of people who bore horns similar to Ico's, specifically the Idol Doors and a statue outside the Tower. This tells us that these people must have been around when the Fortress was built. Since the statue is right outside the Tower, this may suggest a link between those people and the Casket Chamber within. The events of Ico also suggest a link, in that it is specifically children with horns that the Queen needs as sacrifices. Furthermore the fact that the Idol Doors bear images of children in pose not unlike Yorda's when she is in the cage suggests that a bad fate awaited such people in the Fortress.
The Spirits seen throughout the Fortress in Ico tell us that there were quite of number of these people around at some point in the past, presumably back when the Fortress was inhabited. I mention in the magic analysis that it is possible that anyone who bears the horns becomes a Spirit when they die. This may not be the case, but it is certainly possible that it doesn't require much to cause them to become Spirits - perhaps random chance or the influence of someone who could use a new breed of followers. If we assume that nothing so grand as a sacrifice in the Casket Chamber is required to turn these people in to Spirits - another possibility I mention, but do not believe makes sense - then several parts of the story come together.
This assumption illuminates a progression of the story which parallels that of a Light Magic Queen turning to Dark Magic, and in the process dooming her domain to ruin. If the Ancients were indeed actively seeking to stop the spread of the Spirits birthed by people with horns, it stands to reason that in their time such Spirits would have been relatively rare. However, by Ico's time, rather than efforts to curtail their numbers being made, the Queen likely encourages their existence, resulting in the Fortress being overrun by the very creatures that people were sacrificed to prevent from coming into existence.
This pattern of persecution which was possibly warranted but nonetheless unfair actually carries on into Ico's time thanks to the Queen. Just like his ancestors before him, he is sacrificed because of the mark he bears. The motivation is different this time of course, but what difference does that make to the sacrificed? If Ico's people truly are descendants of some element of the Ancients, they may still carry a distrust of anyone with horns, and it may have been by playing on this distrust that the Queen convinced them that such children were bad omens who should be sacrificed. The apologetic sacrifice of one of their own by the men at the beginning of Ico may be an unwitting re-enactment of actions carried out by their ancestors centuries ago. It would seem that even the ruination of the Fortress was not enough to free those with horns from the burden they carry. However, Ico's actions may offer some hope.
Ico's actions may free any future horned people from sharing his fate. Since the Fortress is gone and The Queen is dead, the last traces of the race who would see them sacrificed are now gone. I mention in the character analysis that he gains the opportunity to face his nemesis in person, and by fighting her frees himself literally and metaphorically from his curse, in that he has lost his horns and the threat to his life. This holds true for any other people with horns as well, what Ico may have done is remove this burden, if not its mark, from everyone like him.
Written by Crumplecorn