1 How does one get a stronghold?
2 Are they in multiplayer only or what?
3 What does one profit from them?

Depending on what class of char. you are, you will have the opportunity to get a strong-hold for that class char. You must fulfill a certain quest, in which you will be rewarded the stronghold for your work. If you do all the sub-quests, you are 100% sure to run across the one that pertains to you. Don't sweat it, and just run around fulfilling quests, and when you get to the house that you will soon own, they will let you know how to obtain it.

Now, if you woud like to know immediatly, tell us what type of char. you are playing, then we can tell you specifically where YOURS will be in the game. (If you wish)

im a 11th level swashbuckler soon to be dualed to a mage. If i do that quest now i get a thievesguild right, but is it that or planar sphere after dualling or will i get them both.

how will i get that thievesguild and what will i get