On the entrance of the cardinal, one would have said that an invisible magic thread had suddenly drawn all glances from the stage to the gallery.

【翻译】一进入红衣主教教堂的入口，人们可能就会说，有一根看不见的魔法线突然把舞台上的所有目光吸引到了画廊处。

one 表示“人们”，指任何人。

would have done 有两种用法：

1. 表示与过去事实不符或相反的虚拟语气

2. 表示对过去情况的推测。本句就是这个推测用法。

表示对过去发生事情的可能性推测。例如：

（1）His language would have done no credit to a present-day BBC announcer. 他讲的语言当时可能会降低现今英国广播公司播音员的身价。

（2）People are recycling many things which they would have thrown away in the past. 现在人们都在回收他们过去可能会丢弃的东西。（2014安徽高考试题）

（3）Well, the site was close to the Murgab river, which they would have depended on for their water. 那地方紧靠穆尔加贝河，他们当时或许要靠这条河用水。