The Front Door is not necessarily a demense of the House, but is difficult to classify as anything else other than a go between for the other demenses of the House.
The Front Door is a dome, containing few sources of light (causing it to be difficult to see in it for the most part), half a mile in diameter, containing hundreds of entrances to demenses of the House and the Secondary Realms. This means that the Front Door can be used to get to different parts of a demense quickly, however some domains only contain one usable entryway e.g. The Lieutenant Keeper told Suzy the only portal he could open to the Great Maze was in the Citadel, however it is possible that this was due to a lack of authority or that at the time the New Nithling army led by the Piper meant that other doors into the Great Maze had been sealed to prevent their spread into the other demenses of the House.
As well as doors for general use, the Door contains at least one postern (Monday's Postern) and can so be assumed to contain Posterns for the other days.
Not all entrances to the Front Door lead to all places. Indeed some only lead to a single place. For example when Arthur enters the Front Door from under the mill in Grim Tuesday's Grotesque's lair, he can only access the far reaches from it as the Lieutenant Keeper explains to him that this particular door is a direct connection to the Far Reaches, and as such he could only hold Arthur in the Front Door for a short time. Travelling through most entrances to the Front Door places the entrant into the large dome, well as this door bypassed that and took the entrant straight to the Far Reaches.
When travelling through the door, one must have an object of power, for example, a Key, or wait for the asisstance of the Captain or Lieutenant Keeper, as Suzy had to do in Sir Thursday. Arthur was able to travel through the door first by using the Lesser Hand of the First Key and then by using the power of it that resided in him. Sunday's Dusk, aka "The Reaper" was able to use his sythe and the Lieutenant Keeper uses his sword.
The Door is guarded by the Lieutenant Keeper and until the Architect disappeared, the Captain Keeper. The Lieutenant Keeper was fataly wounded whilst protecting the Door in Lord Sunday. He managed to give Leaf his sword and as such the duty of becoming the Lieutenant Keeper while she was being carried to the Incomparable Gardens by Sunday's Dusk. Following Leaf's brief period as Lieutenant Keeper, Fred Initial Numbers Gold claimed the title but who currently holds this post is unknown.