How can I set a milestone?
In Twproject milestones are always related to a project or task. Both start or end date can be set as milestones. See 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. See 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 task list page.
Does a milestones create time constraints?
Yes. Once a milestone is set, every change on task tree structure that requires time shift on parents are blocked. This allows to delegate sub task to other resources without compromising the main project schedule.