so i quickly jumped into youtube to see what it looks like.
i think it's basically up to what you want to have as an end result.
Is it supposed to be a single player TD? Multi Player? PvE? Tower Wars?
Do you want just one level or split up the map and make several small levels with "simpler" terrain, or go for one larger level?
Height change is still an option as long as you put ramps or just use normal terrain deformation.
As it's a tower defense, terrain is not the most important part in my opinion, just make it obvious how creep routes work and fill only the outer areas with some details
If you want to keep the appearance similiar to the game, i think the lordaeron summer tileset as base works best, add and delete special tiles as necessary
PS: Also note that with the recent change in copyright for maps, you have to be careful when copying existing games into wc3.