I think Jenni's instincts are correct and that the right word here is probably terrace. But if it's right the first time, it's also right the second, because they must refer to the same structure.
The fact is that the semantic range of "balcón" is wider than that of "balcony". A balcony is always on an upper floor, I think, but "balcón" can mean a "mirador", and I think it can denote a projecting ground-floor platform commanding a view, which we would call a terrace in English. The chapel is clearly no longer there, so it's difficult to see how the "indicios de lo que debió ser un balcón de madera" can refer to a balcony above ground level. I think this is probably about Parada de Sil, where they've found "los restos de las cimentaciones de la capilla" including "una curiosa estructura que define un balcón cara a la explanada de las sepulturas" (
http://www.paradadesil.es/svitor-e.html ). This structure must be at ground-floor level, so we would call it a terrace rather than a balcony, in my opinion.