Architect
The Architect is an NPC found in the second floor of the Crypts level. Once rescued, his building can be constructed in town for 25 stone and 25 crystals. Once constructed, the Architect offers six unique buffs that can be applied before starting a run. After speaking with the Architect in town, he can then be found outside the Dark Citadel in the first level of the Forest. He offers to open the door to the tower, giving the player the ability to skip levels for the current run.
Cost Multiplier

On the Architect buff selection screen, there is a cost multiplier in the bottom right of the screen that reads "Mul: 100%". Selecting multiple Architect buffs raises the cost multiplier, increasing the gold and material cost of each buff selected. Upgrading the Architect to tier 2 lowers the cost multiplier.
For example: At Architect tier 1, selecting two buffs raises the cost multiplier to 141.35%. This is a 41.35% increase in the cost of both buffs selected. In this example, selecting both Far Sight and Toppled Towers would result in a cost of: 2827 gold, 14 wood, and 7 stone.
Architect Tier 2
Upgrading the Architect to tier 2 requires 50 stone and 50 crystals. Tier 2 unlocks additional buffs that can be added to a run and lowers the cost multiplier of selecting multiple buffs.
Trivia
The Safe Corridors buff will remove the shadow explosions that trigger throughout the Dark Halls, as those are generated from invisible traps that reset a few seconds after going off.