TL;DR
The project manager’s responsibility is to organize the work of product development. This is the project lifecycle. Read another post from the project management series. The post is based on a remarkable book written by Johanna Rothman, Manage It!
Here is Johanna’s definition of the project lifecycle:
The project lifecycle is a way how the project manager and development team organize project work.
I had seen the project lifecycle as an obstacle, and now I understand why. Either project lifecycle could not fit into our development team, or the project lifecycle was wrongly used. I worked in a waterfall, agile and iterative lifecycles.
So far, you have your project charter and plan. While you were creating those two, you got some information that could help you to select the project cycle. If you received a Bible sized set of requirements, you should go with the Waterfall lifecycle. But if you see that defining requirements will an ongoing process, you could go with Iterative or Agile. Your team is a startup company, go for Agile!
When you have a prioritized list of project drivers and customer expectations (your constraints), it is much easier to choose the project lifecycle. The main goal is to mitigate project risks with the most appropriate project lifecycle.