Autoplay takes the grind: it picks a live creature on the floor and fights it, over and over, and steps through the portal when it opens. It unlocks on floor 6, takes no action you could not take by hand, and never enters a boss den.
1The AUTOPLAY button
AUTOPLAY sits at the bottom of the floor. Click it to start; click again to stop after the current battle. The small gear beside it opens the autoplay settings.
2Its settings
Under GAME SETTINGS, BATTLE & AUTOPLAY holds what autoplay does on death (stay or go down), when it challenges the boss, what it does out of keys, how it targets, when it uses the ultimate, and whether it prioritises buffs or debuffs. They are part of your save.
3Floor transits
The portals at the bottom corners move you between floors. ASCEND is sealed until you have the floor's wins - the line under it counts them down; DESCEND is always open. Autoplay takes the open portal by itself and stops at a boss den.
Screenshots are taken from the live game by tools/tutorial_shots_b.py (dfmc-client) and regenerated after interface changes. The rules behind each screen: system-autoplay, system-stage.