This paper introduces Alpacas, a language and collection of algorithms aimed at critical system architecture modelling and safety assessment. It allows to study the effects of random and systematic faults on complex critical systems and their reliability. The underlying semantic framework of the language is Stochastic Guarded Transition Systems, for which Alpacas provides a feature-rich declarative modelling language and algorithms for symbolic analysis and Monte-Carlo simulation, allowing to compute safety indicators such as minimal cutsets and reliability. Built as a domain-specific Language deeply embedded in Scala 3, Alpacas offers generic modeling capabilities and type-safety unparalleled in other existing model-based safety assessment frameworks. This improved expressive power allows to address complex system modelling tasks, such as formalising the architectural design space of a critical function, and exploring it to identify the most reliable variants. The features and algorithms of Alpacas are illustrated on a case study of a thrust allocation and power dispatch system in an electric vertical takeoff and landing aircraft.