So to answer your question, Simon, yes, he can build it. He says it will take some time and Etta says it would be easier if he didn't have to do it alone, right? "If you had your team with you to help you?" She reminds him that he was trapped in amber with them and asks if he remembers where that was.
He stares at her intently without answering and then finally says, "You," in recognition. And then, in the distance, the sound of an elevator dinging. Walter can't believe they didn't disable the internal alarms and he asks if they're idiots, so it's good to know that his ingratitude cortex wasn't permanently damaged by the amber.
He heads over to a wall and puts his palm in a certain spot, which lights up underneath his hand and a door slides open in the seemingly seamless wall. "Neither Belly or I ever left ourselves only one way out of a room," he says.
They head out the secret exit and the loyalists aren't far behind. One of them hilariously puts his hand on Walter's chair and says, "Warm!" They were just there! So how'd they get out? The Observer either scans through time or some shit or just figures that once you eliminate the impossible, whatever's left -- no matter how improbable -- is the answer and he pulls out his gun or phaser or whatever the hell and warns everyone to stand back, as he aims it at the wall with the secret exit.
We see our heroes running through a corridor and the shockwave of the Observer's gun sends them reeling, so what a total hosejob that we don't actually get to see him destroy it! They recover and keep going, with Walter yelling "Two lefts and a right!" to Simon and Etta and then purposely letting them run on while he sneaks off somewhere else. It never occurs to Simon and Etta, whose combined ages probably don't match Walter's, that maybe they shouldn't let the old guy WHO HAS BEEN IMMOBILE FOR THE PAST TWENTY YEARS try to keep up with them. They only realize he's not with them when they arrive at the door and can't open it. So they double back.
Fortunately, the Observer and his men are employing the television chase technique of Marching, Not Running, so I imagine they've been falling behind steadily.
Simon and Etta find Walter in another lab, fiddling with some device and they urge him to come with them, but he just asks Simon for his watch.
"Resistance must take place at any opportunity," says Walter, putting this doohickey into that thingamabob. "We are insurgents and this is anti-matter." Now it's really time to hightail it out of there, but not before having a quiet "holy shit" moment.