To be honest, I'm not even sure. I didn't fully understand what they were at all. Maybe it was explained but really missed something.
Also, I thought the episode was really well-written. You really grow to hate those slavage guys except for the and it was a good episode overall. I think the library was my favourite part of the TARDIS, especially when Clara read the book and said "so that's who...". Nice foreshadowing for future events...although I'm not sure the series would be able to continue if we ever learned his name as an audience.Oh, and I like the touches like Encyclopedia Gallifrey being in bottled form. Imagine having a drink of that at a party!
And then there was the Eye of Harmony. That confused me because I thought the TARDIS ran on time energy. But looked it up, it USED to use the Eye of Harmony as a 'link' to the Gallifreyan original but since the original is gone then it uses time energy, thus the rift is needed.
My only complaint was the title of the episode. I guessed it as soon as the trailer played last week, it could have been a little better. Oh well, we got some nice TARDIS scenes and a creepy monster.
It still irks me that we STILL don't get Cybermen next week. come on, we've all seen the photos! How much longer do we have to wait!
Other notes:- The swimming pool's back AND we got to see it. That was cool!
- the library...my god..,,I was looking at the stuff hidden around, like River's cot, and trying to work out if there were any other old who trinkets I might know of.
- bit of a shame we couldn't see the old console like we did the last time the TARDIS was invaded, but time echos are good too.
I'm starting to think the TARDIS should give herself more natural defences from being damaged. She's a living machine after all.