One special characteristic about Sen Mag Plateau is that Guild Battles are held once a week, on Saturday, at this location.
It is one of the two faster ways to get to Emain Macha. The other way is through Osna Sail from the west of Dunbarton or you can take the long route going through Taillteann to Tara or go through the Abb Neagh Housing Channel, which puts you next in between the Advancement Arena and Coill Dungeon.
This place held some of the bloodiest batles in the first war against the Formors and Humans. The reason that there is little life here is because the Glas Ghlabehiens killed here spewed out toxic acid and darkness making the land uninhabitable for centruies.
Fleta is the only NPC that can Repair Items with Passive Defense and some items that came from events like the Shooting Star Robe. Her Repairing Rate is 98% Fleta and Rab only appears certain times during the day in game. Here's a table for when she spawns:
She also has some side quests that can be done for a few coupons and you need to befriend her to get rank C rest.
In the Sen Mag Plateau are several bears (Brown Bears, Red Bears and Black Grizzly Bears), and occasionally the Field Boss: Giant White Grizzly Bear will spawn along with Black Grizzly Bear minions.
Near the Sen Mag Plateau is the Housing Channel (South), Peaca Dungeon, (Near Northwestern) and Emain (West)