The Forces of Light and Dark in Ico
The Orbs and Reflectors
Sitting behind the arenas on the east and west islands are metallic devices known only as the Reflectors. Each one is paired with one of the Orbs sitting above the main gate. The Reflectors, when aligned with the Orbs, gather power and send a beam of light to them. This activates the Orbs, which in turn energize the Main Gate using the same power Yorda possesses.
The Reflectors & Light Magic
The Orbs, when activated, possess Light Magic. They seem to draw this power from the Reflectors, which remain active along with the Orbs. The power coming from the Reflectors takes the form of a beam of light, but this is not an ordinary beam of light. For instance, it should not be visible in the middle of the day unless you look straight at the reflector. This is likely another, more passive, form of the Light Magic. Assuming this is the case, there are two very significant points to consider with the Reflectors.
First is the apparent mechanical nature of the Reflectors. This alone does not tell us anything specific, but it does make a general statement about the nature of Light magic. The most 'magical' parts of Light Magic are seen in Yorda and the Sword. Yorda has apparent 'magical' abilities, and the Sword is imbued with this same power, making it seem to be a 'magical' sword, complete with glowing runes. The Reflectors however seem to be mechanical or technological, and yet they are also a source of Light Magic. This suggests that Light Magic might not really be 'magic' at all, in the supernatural sense of the word. For comparison, Dark Magic involves things like Spirits appearing out of portals, the Queen appearing and disappearing and her purely magical attacks. Juxtaposed with this, Light Magic seems more to be some esoteric sort of energy which some people and items are able to control. Light Magic is undoubtedly 'magical' in nature by our standards, but the Reflectors suggest its true nature is a little more down to earth than 'real' magic.
The second point is a question they raise; from where do the Reflectors draw this power? Here the name can give us a hint. The Reflectors do not 'reflect' like a mirror, this is fairly obvious because they aren't actually reflective; they emanate a beam of light. I think they 'reflect' in a more conceptual sense, they take this power from some other source and focus it into a more coherent form. However there is no sign of a source, magical or otherwise, for the power they 'reflect'. They seem to simply draw it directly from the world around them, and I think this is exactly what they do. Going along with the suggestion that Light Magic is not a power from a supernatural source, this raises the possibility that Light Magic has a natural source, that it is a part of the normal world. This leads into a full theory on the nature of Light Magic, which I go into in detail elsewhere.
The Orbs & The Design of the Fortress
The next part of the system to consider is the Orbs. Staying for the moment with what the system can tell us about the nature of Light Magic, the first thing to note is the physical design of the Orbs. While the Reflectors supply the beam of light which is somehow related to Light Magic, it is the Orbs which actually control the signature lightning-style magic. They bear no resemblance to the Reflectors and, being covered in runes and glowing with magical power, appear far closer in design to other devices related to Light Magic, like the Sword and the Main Gate. While there may be more to the overall nature of Light Magic, the power Yorda and the Sword hold is obviously directly related to the more magical parts of the fortress.
I mentioned earlier that the Orbs and Reflectors likely have a purpose other than being a giant locking mechanism. The first point to consider is the sheer size and prominence of the system. The Reflectors are located on separate islands to the extreme east and west of the fortress, each sending a beam of light half the length of the fortress to huge devices positioned directly above the Main Gate. It is far too big and complex to simply be a way to make the main gate secure, though it would surely do that well. It is more likely that the gate was built to suit the Orbs and Reflectors, rather than the other way around. In this case we need to look at the whole thing the other way around. Rather than the Orbs and Reflectors being designed to make it difficult to open the Main Gate, the Main Gate was designed so that it could not be opened without the Orbs and Reflectors being active. It is likely that they, along with the massive gates and colossal retractable bridge, are some kind of defence for the fortress. Requiring that they be active before the main gate can be opened means that the fortress cannot be entered while these defences are inactive.
To understand the implications of this, we need to take a brief look at one of the other devices in the fortress; the Idol Doors. As with the Main Gate, these require some use of Light Magic to open. If the Idol Doors are activated when there are Spirits nearby, they use Light Magic to destroy the Spirits. We see the Orbs do something similar when Yorda tries to leave; they don't kill her, but they do prevent her from leaving, showing that they can project their power out along the bridge. It is not a major leap of logic to conclude that the Orbs can also use this power the way the Idol Doors do, that if there were any Spirits in the area of the main gate while it was open, the orbs would destroy them as the Idol Doors do.
The significance of this is that it tells us something very important about the Fortress; that it is designed to protect against the Spirits, or something like them. Given what the Idol Doors are able to do given the small amount of power Yorda gives them, the Orbs would be incredibly effective at stopping any Spirits from even approaching the fortress while they are active. Even if some did get past this defence, each section of the fortress is separated by the Idol Doors which also destroy any nearby Spirits when opened. Being able to infer this aspect of the Fortress' design in interesting in itself. What makes it really special is the fact that in Ico the Fortress' Queen is a Dark Being, and it is inhabited by the Spirits it seems designed to destroy. This opens up into a much wider discussion about both the original purpose and possible history of the Fortress, which is a topic of its own.