We are controlling Desmond. Talk to other people.  Later you will go downstairs to train some new skills. 
We need to activate four buttons. Do not pay attention to the hallucinations. The first and second buttons are on the lower level (on both sides).  Two others are on the upper platforms (on both sides of the magazine). Get to them using shelves and the ladder. 
When you will turn on all the buttons, go back to Lucy and follow her Some strange thing will take place in the corridor. Surprisingly we will control our old friend - while entering the castle.  Probable target will appear - we are chasing him using roofs and walls. 
You will get to the tower - the doors will be closed.  Climb up. At the top, watch some mysterious events.  Then jump into the abyss.
Desmond will wake up, so go back to your task. Speak with members of your team.  When you will be prepared, sit on the couch and use the Animus.