Validate the primary assumptions.
This is the stage you have the highest flexibility to change the solution implementation at a lower cost. Once the product passes this stage, changes become costly due to the involvement of other stakeholders such as customers. You should make use of this stage to validate all the assumptions (technical, requirements, business) your team made during the previous stages.
Every requirement should be treated as an assumption with a plan to validate. Such validation can come in the form of MVPs, demos, user testing, architecture POCs, etc. During the build stage, you should consider onboarding pilot customers as testers to validate the primary value propositions.