If you have ever played Herzog Zwei for the Sega Genesis, then it's like that, but better. I like games that are customizable like that. Imagine SimCity with war. Imagine VTOL aircraft, and flak guns to knock them down. Warships with great range, and submarines to sink them.
Torpedo carrying planes take care of shipyards, but watch out for anti-air ships.
Blast your enemy from miles away with unguided artillery. In addition to the many units the game comes with, the TA site offers many an excellent unit for download, in addition to more maps, and a fantastically easy to use map maker so you can make your own. TA requires a Pentium 90 to run, but you will be happier with a P-120. Of course, these days, you probably have twice that anyways. If you are gonna play on larger maps, you will want 48 megs of ram at a minimum, but 64 is probably a nice comfy number.