There aren't many RTS games on Newgrounds, for good reason; they're hard to make, even in 2D. They take time to balance and design. It's rather incredible that Age of War was made in such a short period of time, and has stood the test of time as one of Newgrounds's best games. I play this game to relax fairly often. I have played a LOT of this game. So much, in fact, that I've created a few modifiers you can try if you've beaten the game and want an extra challenge. Not all of these challenges are possible on all difficulties.
-Fully build tower in Caveman age, then win, taking no damage (Normal, Hard, Impossible)
Yes, you can complete this challenge even on Impossible difficulty. Like all of these challenges, I did it myself first to see if it was possible. Especially during the Caveman and Sci-fi ages, you'll need to send out a unit or two just to keep the enemy off your base, but be careful! If you're trying to do it with no damage taken, keep an eye on your opponent's base to see if they build an IonRay (the blue-eyed laser turret), as this can hit your base directly if one of your units dies too close to their base. On higher difficulties, the hardest part is the "no damage" modifier, so you can leave it off if you want. Apply turrets liberally and you can survive your Caveman age.
-Win before Sci-fi age taking no damage (Normal, Hard)
Mostly self-explanatory. This one is mostly about spamming ranged units, but be careful! Like in the challenge above, enemy Mousquetteres during the Colonial/Renaissance age can ruin your whole production in an instant if you can't counter them fast enough. Luck is a factor here, as well as deciding when to hold off and produce barely any units, and when to launch your attack of many units. In most ages, resist building your elites and instead focus on the tried-and-true strategy of a melee unit backed up by many ranged units.
-Win without creating a single unit, using only turrets and Specials (Normal)
This one is pretty fun. It becomes more like an Idle Game than an RTS. Choose carefully when you use your Special (like when more than one Dino Rider is on the screen during Caveman age, for instance). You cannot avoid taking some damage here. The Renaissance age needs to be skipped quickly, since the Holy Spirit ability is useless without units. Once you reach the Sci-fi age, the last turret, the IonRay, can sometimes hit the enemy base directly. If you get unlucky with the spacing of the enemy units for too long, you could always just give in and buy a Super Soldier or two to finish the job, but it's perfectly possible to pick your timing with the orbital laser or just leave the game running with full IonRay turrets until you see that sweet Victory screen.
Enjoy! Send me a message if you're unclear on the rules or have your own ideas.