Go through the Prison, you come to the library, get the 2 life upgrades if possible, then
you come to the Mechanical tower, Kill the Brute, then you come back to the central
hall then head to the throne room.

NOTE: There is a Life Upgrade in the Southern passage.

i have switched from presnt to past three times now,and still nothing

Just go back to garden, and change from past to present using the portal, when returning
to top, the Dahaka will slam prince through a door, thats the start of a prison area which
you need to go through.

how to return to throne room(past)