MB01 - Jerusalem | Memory Block 01 Assassin's Creed (PC) Guide

Status: Retrieve the Artifact from within Solomon's Temple.

Move forward jumping on the beams. You can do it pushing right trigger/RMB (High Profile) with X/LMB. Assassinate the guard and climb down the ladder.

You screwed so there is nothing to do but to run away. Climb the scaffolding and go east jumping onto the platforms. Now climb the wall (right trigger/RMB + X/LMB)