Three machines to break: the Cinder Automata bosses
The Siege Brute, the Colossus Walker and the Hive Queen, and how each one is meant to test a different part of your base.
Every run builds towards three bosses, at levels 10, 20 and 30. After that, Endless mode lets you see how long you can last.
Siege Brute, level 10
The Brute walks straight at your core and ignores your turrets unless they are in its way. It hits buildings much harder than other enemies, so it is a test of your walls and your focus fire. If it gets stuck on rocks or trees, it simply crushes them.
Colossus Walker, level 20
A towering machine that steps over defences and shrugs off small arms. Heavy enemies have flat armour, so a pile of cheap bullets will not do. You will want better ammunition and turret upgrades by then.
Hive Queen, level 30
The Queen keeps spawning drones. Break the swarm first, then break the Queen. Some enemies are insulated against Storm coils, so lightning is not the answer to everything.
The visual rule I settled on early is that your own robots look clean and freshly built, with blue and orange trim, while the enemies stay rusted and feral with red glows. In the middle of a fight you can tell at a glance whose side something is on.