That was my first thought too, when I read these words for the first time but it appears that they are more like streets. But I could also interpret too much into what I've read.

[He was familiar with sewers, thanks to his father. If these 'tunnels' would turn out as one ancient sewer system he would at least know a couple of things about them. And where sewers are, there is life - or was life, in this case.]

I doubt that there is much need of sewers here. But if they are there, maybe they found a way to hide them or make them look like something else? I guess here everything is possible.

[Tyrion made a short pause in which he was organizing his thoughts.] I guess that would be good point to switch the topic of the research. But before I oder we will do that, I need a break. What about a walk around the building?