Factory Methods in the Aggregate Root are also a good place for invariants.

In a Domain Model with Forum and Post Entities, where Post is an aggregated part of the Aggregate Root Forum, publishing a Post could look something like this:

class Forum
{
    // ...

    public function publishPost(PostId $postId, $content)
    {
        $post = new Post($this->id, $postId, $content);

        DomainEventPublisher::instance()->publish(
            new PostPublished($postId)
        );

        return $post;
    }
 }

After talking with a Domain Expert, we came to the conclusion that a Post shouldn't be published when the Forum is closed. This is an invariant, and we could force it directly on Post creation, thereby preventing an inconsistent Domain state:

class Forum
{
     // ...

    public function publishPost(PostId $postId, $content)
    {
        if ($this->isClosed()) {
            throw new ForumClosedException();
        }

        $post = new Post($this->id, $postId, $content);

        DomainEventPublisher::instance()->publish(
            new PostPublished($postId)
        );

        return $post;
    }
}