Time, Killstreak & No-Hit Doors

Timed Doors
Timed doors appear in few transition levels (levels that are in-between two biomes). If a timed door is not reached and opened within a set amount of time in a run, it will be locked and unopenable. Time spent in the beginning area of The Prisoners' Cell, the Collector's Room, in shops, treasure and lore rooms does not count towards timed doors. Behind each door are some gems and jewels, which turn into gold when picked up, as well as 20 cells. Some timed doors also contain a blueprint and a 3-choice item altar that consist of either 3 items or 2 items and 1 amulet. The items in these altars always have the same quality, depending on how many BSC are active.

Perfect Doors
Perfect Doors appear in most transition levels (levels that are inbetween two biomes), sometimes even alongside Timed Doors. There are 2 types of Perfect Doors. Killing mobs that do not reduce the curse counter will not increase the kill counter, but getting hit from them or traps will reset the kill counter. The contents of the Perfect Doors following biomes are the same as timed doors. Perfect Doors that follow boss fights are different: they contain one Legendary item instead of a 3-choice item altar. The rules for item quality are the same as for the item altars behind Timed Doors. Being poisoned, for instance by pools in the sewers, with a no-damage or "poison heals you" buff will not count, though the poison DoT symbol will appear over your head as normal
 * The first type is in transition levels after a biome and requires you to reach a set amount of kills in a row (30 or 60) without getting hit. Even if you get hit before the start or after the end of your kill streak, the door will still be open.
 * The second type is located in transition levels after bosses, and requires you to defeat the boss without being hit once.