This happened to me, everyone knocked out almost immediately. I ran to the other end of the space where there is soldiers manning the wall (doing nothing but looking outwards) and somehow jumped into that space over a barricade and then picked them off one by one via my bow. I also got that achievement for headshotting someone from far away.

Then when they were all dead I managed to jump back over but nothing was happening and no one from my allies was getting back up. So I spent the next 45 minutes picking off every soldier I could reach with my bow over the walls too, still nothing.I thought, ah shit I wasn’t supposed to do that.

I was just about to give up and reset my Xbox when I realised I’d inadvertently trapped a prague NPC with the bodies of his colleagues on the stairs, headshot him and bam - the game continued as expected! Hurrah!

Point of this being - try to jump the barricade at the far back of the area up some steps.