Orcs Must Die! Unchained is making the tower defense series free-to-play later this year, but first, you can try it out in beta form. The closed beta will begin on Friday, June 27. As compared to the alpha test, the beta will introduce a new map, three more heroes, and the deck editing system. Heroes will be rotated throughout late June and early July.
You can register for the beta at the official site. If you played the alpha, you'll be grandfathered in. You could also buy your way in by purchasing a Founder's Club Pack, since that still gives you alpha access. Those run between $19.99 and $149.99.