Remove building times and come with a finite amount of resources perhaps?
That way, your base building will hit the ground running.
Also, you can sequence things as in Ground Control where reinforcements are shipped in waves, thus, there is no incentive to fire a first volley of troops to catch your opponent unawares (shifting the focus towards the 'puzzle box' of trying to maneuver units around enemy's units).
One thing that might become stale however: if both bases are sturdy from the get-go, then the game will be one of attrition as the defender will always be advantaged and no player will have the incentive to fight. In this scenario, I would recommend using a different approach...
What if all units are AI controlled and behave like a mob? That way, you can always focus on your base, and not worry about each player avoiding to send their troops.
As the game progresses, mobs on both end would become more powerful/numerous, so that only the 'best base' would stand. It looks a lot like a mirror tower defense now doesn't it?