gather the seals
After a couple of beginner missions you get symbols on your map which look like 4 arrows pointing at a dot. You go to that location and there you can recruit assassins. You have to take over templar dens to unlock more recruit slots.
By solving the glyph.
multiplayer
You play the game!
unlock certain achievements
A huge hammer
u cant only in multiplayer
Yes.
find 6 keys of romulous
You have to pre order the game
if you mean in brotherhood u unlock it by destroying all of Leonardo's war machines and going to one of his benches