I agree with Panda.
Attack:
Champions as meat shields are pretty darn useful. Trebs become the muscle of invading forces, so upgrading them should be a priority (as well as meat shields to keep the trebs alive). Archers and crossbowmen no longer have much use (at least in my hands). Templars are great, Grens nice. Get your good heros up the highest level you can (and of course have that 2nd hero slot unlocked). Max out your army camps. Might be more important than any other upgrade.
Defense:
Here is where I might disagree with Panda somewhat. I'm not sure if maxing it out is necessary, but definitely, the Archer towers are your backbone. I was slow upgrading them on my main account and regretted that later. Max them out. Ballistas, handy to upgrade, but I might do my Crossbow towers first.