Gallifrey

Prior to its destruction, Gallifrey was the homeworld of the Doctor and other Time Lords, located in the constellation (galaxy?) of Kasterborous, at galactic co-ordinates 10-0-11-0-0 by 0-2 from Galactic Zero Centre. It was removed from time and space during the Time War (DW: The End of the World, Dalek, The Runaway Bride)

Defences
Gallifrey appears to have possessed surprisingly few defences from invaders. This may be due in part to the transduction barrier, which covered the planet and which was almost completely impenetrable by outside forces. However, on the few occasions when a hostile force has penetrated the transduction barrier, the Time Lords appeared to be unable to competently defend themselves. For example, when a small group of Sontarans successfully managed to bypass the transduction barrier and proceeded to slaughter any Time Lords they encountered, they nearly succeeded in conquering Gallifrey until the Doctor intervened and used an ancient Time Lord weapon called the D-Mat gun to destroy the Sontarans (DW: The Invasion of Time). (The D-Mat gun should not be considered a normal part of Gallifrey’s defences as its existence was largely believed to be a myth by the Time Lords and was thus not in regular use.)

Another example of Gallifrey’s reliance upon the transduction barrier for security occured when the Daleks bypassed Gallifrey’s transduction barrier and exterminated several Time Lords, including the then current President (DW: The Apocalypse Element). In both this case and the aforementioned Sontaran invasion, the Time Lords relied upon their guards to deal with the invaders, only to find out that the guards' weaponry (see Staster) was ineffective against their attackers.

External link
Gallifrey website which mixes elements established in various Who media with the webmaster's own theories and speculations