I’ve started watching Merlin recently, and with episode one a thought sprang forth as they do (mainly because Eve Myles pops up in the pilot).
So, Camelot is just spewing over with magic. Or what we percieve to be magic. Some people seem able to tap into the magic, like Merlin. But what if its something more? What if over time this “magic” become more concentrated and is a spill over from something far more powerful, say the enrgy of a time rift?
Dragons? Large reptilian aliens attracted to the powerful hotspot and possibly in touch with timey-wimeyness. All the nasties that seek to use magic for darkness? Humans and aliens alike attempting to abuse rift energy.
That poor singer played by Eve Miles (seriously I can’t remember her name) was killed at the start of the rift’s most recent power spike, brought on by Merlin’s presence, and by this enrgy which is considered magic. Its caused a type a geneological echo that allows here to reappear and once more gravitate towards the energy. Eventually the Doctor suggests this, to explain how Eve Miles was in Cardiff with Dickens as well as modern day Cardiff with Torchwood.
This means that present day Cardiff stands on what was once Camelot.
So thats my theory. Camelot was built upon a time rift and eventually Cardiff was built over its ruins and the rift is now monitored by Torchwood.
Whether Ianto is a Once and Future King fanboy is still up in the air.