How can I set a milestone?
In Twproject milestones are always related to a project or phases. Both start or end date can be set as milestones. https://twproject.com/support/using-twproject/projects-and-tasks/
Are milestone approaching notified?
Yes, by default 3 days before the milestone. You can eventually change this value or even ignore the notification. https://twproject.com/support/using-twproject/projects-and-tasks/notifications-subscriptions/
Where can I see my milestone?
Milestones are visible in the summary bar at the end of “get things done” page.
Moreover you can use “next milestone” pre-set filter on project list page.
Does a milestones create time constraints?
Yes. Once a milestone is set, every change on project tree structure that requires time shift on parents are blocked. This allows to delegate phases to other resources without compromising the main project schedule.