The Web of Time, within the Space-Time Vortex, was constructed by Rassilon and the Other. Its creation made it much easier for the Time Lords (and any other speicies) to travel in time but it also made it much harder for anyone to change history. The Web of Time is the structure that all future races used to build their own histories on. In conjunction with the Web of Time, the Eye of Harmony was used to define the physical and temporal laws of the Universe.

An Osmosis Dampener or Transduction Barrier will put a blockade up in the Vortex that will prevent a TARDIS from traveling outside a defined area of the Space-Time Continuum. A single timeship exploding in the Vortex is no danger but the destruction of a fleet of 1,700 timeships could rupture the Web of Time permanently.  (For much more on the Web of Time see Appendix 4)