The solution lies within these two parts, comrade
(1) A standard terrain line in ToT looks like this:
Desert, 1,2, 0,1,0, yes, 1, 5, 5, yes, 1, 5, 3, Pln, no, ; Drt
The important part now is the "no" between the "Pln" and the "Drt" letters. If you change that "no" to "yes" the terrain becomes impassable;
(2) The other important feature is in the "Advanced units fields" (usually this one is at the bottom of the Rules.txt): There are certain flags to be applied for each unit slot - one flag allows that certain unit to "override impassable terrain". This advanced flags section is labeled "G" (General flags).
Hope this was clear enough, otherwise I can only suggest you try the Cradle of Civ - that´s *the* ToT resources top spot on the web