Back to Square One - advice

[dlcatftwin]

Greetings all,
I am currently enjoying

Back to Square 1

.
I am in room 16404 - I am standing on the right side of the stair case (or ladder well if you will) past the zapper, and no where to go. What's my next move, please?
FYI, Jack's picked up so many keys (on the

hardest

route) that he wasn't able to pick up the blue key in the prior room.
Is it needed in Rm. 16404?
Many thanks,
DLC  :-X