The Castle Project is an open-source application framework for .NET, which emphasizes productivity improvements in software development. It provides a collection of components and tools to aid in the development of dynamic applications in the .NET environment. Key features of the Castle Project include Castle Windsor, a powerful Inversion of Control (IoC) container; Castle ActiveRecord, which simplifies database persistence; and Monorail, a web application framework inspired by Ruby on Rails. The project aims to enhance developer experience by promoting best practices and offering reusable, modular components.