Lights are buildings with people still hiding: brighter for more. Yellow gaps in the wall are checkpoints. You are the white dot.
Phone: drag anywhere on the left half to walk. Attack hits the nearest enemy in front of you. The middle button does what makes sense where you stand: search, barricade, gather, heal, feed.
Keyboard: WASD or arrows to walk. Click or Space to attack (the mouse aims). F search, B barricade, G gather or let go of the people you lead, H heal, J serum, E feed (dead), 1 to 6 pick a weapon, Enter to talk, M map, I items.
Doors: a barricade up to thinly barricaded still lets the living climb in. Above that only people already inside can come and go. The dead need it down to nothing.