Empire is definitely set in the 16th century, maybe pushing it back to the late 15th. It would be really out of place in the Napoleonic period. It's not just the aesthetics, but the equipment and the style of warfare was vastly different.
Midaski named a lot of good options: Clash of Empires, War & Conquest, Pike & Shotte, and Hail Caesar. The now out of print Warhammer Historical rules like WAB and Warmaster Ancients/Warmaster Medieval were decent. But if you're interested in those, you may as well stick to the others mentioned before, as they are newer, more accessible, and somewhat based on one of those two older games anyway.
Crusader Miniatures also produce some good rules. The ancient/medieval rules are simply called "Crusader." The PDF download is fairly inexpensive.
http://www.crusaderpublishing.com/shop/home.phpField of Glory seems popular, but I just couldn't get into it. I thought it was needlessly convoluted.
I'm not sure what is still in print/current, but DBA, DBM, and DBR (collectively known as "DBx") used to be pretty popular back in the day. DBA was the "gateway" game. Relatively simple & fast playing -- but maybe not really meaty enough for a lot of people. (All armies have 12 "elements" or units specified by army lists, rather than points. There is a certain rock-paper-scissor relationship to the types of elements.) These rules may all be ancient history (ha!) by now, though. Most of the DBM & DBR players I knew shifted to Field of Glory when that came out.
I've been curious about the Piquet family of games lately, but haven't tried them. People seem to either really love or really hate them. The web site has a rambling treatise on the design theory behind the game, which has fueled my curiosity.
http://www.piquetwargames.com/index.php?main_page=page&id=1