The tower is 100 floors in ten zones of ten, with the Void Apex above. You move between floors through the two portals or the Floor Map.
1The two portals
Fight the creatures roaming the floor (1). ASCEND (2) opens once the floor's streak is complete; DESCEND (3) always takes you one floor down.
2The Floor Map
MENU > FLOOR MAP lists the zones (1). TRAVEL (2) jumps to any floor you have already cleared; BOSS GATE (3) marks the floor that ends a zone.
3Boss rooms
Floors 10, 20 ... 100 are boss rooms: one fight instead of a streak. The challenge button (1) spends one boss key from that zone and says how many you hold - keys drop from the zone's creatures - and beating the boss opens the next zone.
4The Void Apex
Above floor 100 the Void Apex (1) continues the climb to 120: a boss on every even floor and 30 wins to open each one.
5What death costs
There is one realm and nothing is ever lost: a defeat resets the floor's streak and that is all. On Death (1) decides where you stand afterwards - Go Down drops one floor, Stay keeps you on it; Stop Advancing After a Loss (2) pauses autoplay's climb instead.
Screenshots are taken from the live game by tools/tutorial_shots.py (dfmc-client) and regenerated after interface changes. The rules behind each screen: system-streak, floor-codex, bosses, system-void-apex, system-realms.