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.