In Spidersilk the novel most of the intruders were blind and came from a world called Braila. Although there is an actual Braila in our world (it's in Romania), the name of the city is quite significant because Brail is actually what blind people use to read. This is just one of the hidden gems in Spidersilk. There's a lot of clues and associations that lead the readers to believe the intruders were blind and had many forms of aid that helped them give Mark and his friends a run for their money.