I am stuck on a door, wich is glowing red. aside is a yellow cross in the wall. how can I open the door?

thx

Look for some rocks with signs on them. If you have a single cross the probably you have pass by the other two. You may go straight ahead from the door and you gotta see in the left hand an alley blocked by a fence. Make a way trough it and you'll see the rock. You gotta climb on the wall to reach it.

How to activate these rocks? I face them and nothing... gate is still locked.