At the end of the book the Emperor ascends to the golden throne again. Why? As the imperial forces are retreating they describe the mechanicum built tunnels exploding and collapsing. The book made it seemed like these were artificial tunnels forcing an connection to the real webway. So if they collapsed how does the connection still exist? Also, when the Emperor seals the gate he turns off the machinery powering it and they end up looking at a blank wall. Reminds me of SG1 when they close the stargate. So if the tunnels collapsed and the gate is closed what is he doing on the throne at this point? I always though he was holding the gate closed. However, the way it's described it doesn't seem like he would need to do that. Maybe I'm understanding it wrong. Thoughts?
My speculation is that theres just raw immaterium on the other side of the closed imperial webway gate. And even if shut down, it represents a place where the barrier between worlds is -quite- thin, and thusly a weakness should the Emperor cease pressing on it from the other side. What which once has been opened, can be opened again. But yeah, the golden throne was a webway borer, drilling into the deep warp to connect to the ancient webway. Magnus' folly made the human built sections unstable and constantly required the Emperor's will butressing them to avoid a total collapse.
One oddity however. During Rise of the Primarch, it had been hinted that they originally had planned on exiting by the warpgate in the Emperor's throne room, but after Magnus was chasing them, they detoured to Luna as they could not risk the Emperor's safety. Even Magnus complained to Guiliman that he would not reach the golden throne for now, but wanted to make sure that neither guiliman did.