Krak des Chevaliers is an easy day trip from Tartus or Hama, the latter city being the most attractive base from which to visit, if somewhat more distant one-way microbus trip just under 2 hours. Buses also run from Homs and will drop visitors off at the castle. Bus fares are less than US$1 and the trip takes less than an hour each way. A taxi from Damascus cost around US$ 60 round trip also including the driver waiting outside, depends on your bargain skills. Some hotels in Damascus offer bus trips to the castle, but it constitutes a very long day out for the one site.
On the whole, it's best to arrive early - by mid-late morning the castle can be almost submerged in hordes of tourists coached up from Damascus. Sunrise is also a great time to enjoy the view of the castle in a magnificent light. Ditto sunset.