The first Kohan and it's expansion, which was a major improvement to towers, IS still a great real time strategy. The beauty is found in the skirmish mode rather than the campaign. The game becomes even better when played co-op with friends. You may have to play a few games to find the settings that provide the perfect challenge. With some time you will find the balance. The AI can beat you! What a concept.
Give it another try in Skirmish. I'm guessing you played the campaign and became bored quickly. I think you'll be glad you found the disk. The expansion is a stand-alone. It is worth the money if you find you are enjoying the base game. Bigger squads, new heroes, new units, much better towers, etc.