After having been in charge of building a large distributed enterprise-ready web-based system for over seven years, it is time to reflect on that period and share some of the most important lessons I've learned on software architecture and agile software development within the .NET realm. Could I have avoided the monolith in the first place? And what about domain modeling, technical design principles and design patterns? Do they still have merrits? And don't forget .NET coding practices, testing practices, data management and deployment techniques. In other words, what do I think I should do again or differently the next time I need to design a complicated high-performance system.