Played on normal. At first I thought I was going to be in real trouble. Tarth's ranking was at least 100 pts more than mine. I was also surrounded by their cities and troops. I had nowhere to expand to so I knew I had to go to war. I built up my army, got into position and waited for our non-aggression pact to end. (Really do need a counter or something that tells you how many years are left in a non-aggrestion pact).

Once the war begin, I rolled right over them. It was a little too easy. What I noticed is that Tarth had many many cities, too many in my opinion. I only had a two cities with a number of outposts. Tarth should have used more outposts and less cities.

Secondly, every city or at least many of them had barracks, command posts, etc. Tarth must have spent much of their resources on these rather than building an army.

During the war, Tarth had many more troops and they did attack in stacks but still too piecemeal. One small stack, followed up by another small stack. If I were Tarth I would have retreated to a choke point and defended. They kept attacking and throwing away troops. 

I simply took 2 strong stacks and walked through the world razing their cities.

Still I enjoyed and I am going to try challenging next.