The results of an intensive site testing assessment campaign, held at the Observatorio del Roque de los Muchachos (ORM) (La Palma) during four nights on July 1990 are presented and discussed. To evaluate the optical quality of an astronomical site for high angular resolution astronomy our philosophy leads us to make use of as many instruments as possible. The Scidar instrument was adapted to the Nordic Optical Telescope (NOT) focus, whilst free-flying balloons and an instrumented mast were used to evaluate the 'open air' seeing. The results are described following a classical approach in terms of seeing. During three nights out of four, image size was better than 0.7 sec. The mean value, however, goes up to 0.96 sec due to strong turbulent layers appearing during part of one of the observing nights. The geophysics relevant to understanding the genesis of these turbulent layers in terms of the atmosphere stability is discussed. The contribution to image size degradation from the surface layer is almost negligible. From a comparison between seeing measured at at the NOT focus and the seeing for the whole atmosphere, it is concluded that NOT dome and mirror seeing do not significantly contribute to image degradation; it is an example of a well designed system in terms of thermal turbulence. The mean value of seeing attained at NOT focus with Scidar working as a seeing monitor was 0.76 sec. From balloon soundings, quantities related to high angular resolution astronomy like the speckle lifetime and the isoplanatic angle, as well as the integrated content of water vapor relevant to IR astronomy are obtained. The isoplanatic angle is less than two arcsec and, in most cases, the number of well defined turbulent layers is very small, so that multiconjugate adaptive optics is applicable. The mean value of precipitable water vapor content is 3.4 mm with a minimum attained value of 1.99 mm. This points towards the ORM to be a potentially good site for IR astronomy although, clearly, a longer sample would have to be taken for this to be established.