One of the more difficult problems with using open source libraries in the .NET space is the incestuous nature of the .NET open source community. NHibernate has a dependency on Castle.Windsor. Castle.MonoRail has a dependency on NHibernate and Castle.Windsor. Fluent NHibernate has a dependency on NHibernate. TopShelf has a dependency on Common.ServiceLocator which if you are using the Castle stack adds a dependency for you on the Windsor provider for Common.ServiceLocator.