drainage

[Sydney]

As an absolute beginner, I have a very basic and fundamental question: should a terrarium have drainage holes in the bottom? Won't the water stagnate in the base layer (I'm planning to use expanded clay balls "hydroton" and install a small fan for circulation).
Also, with regard to terrestrial orchids, can they be planted directly into the substrate - or is it best to place them in plastic pots, which can then be disguised with sphagnum.