An addictive online turn based strategy game, Tactics Arena Online is the most tactically interesting game I've encountered on the internet.
Although TAO is set in a medieval fantasy style world common to RPGs, TAO's gameplay is more similar to classical tactical board games like chess or checkers. TAO is a one vs. one game played on a chessboard-like battlefield. Each player has an army of units, all of which have unique characteristics and abilities. The goal of the game is to use your units to kill all of your opponent's units before he or she can kill all of yours.
The tactical choices begin before the battle even starts. You can set up a choice of ten units in any way you'd like on your part of the board. Unit selection and placement is one of the most important and controversial aspects of the game, with much argument among players as to the relative power of different setup types. One tactical school advocates bunching all units into one corner of the board, allowing for a powerful defense. A setup of this type best protects the cleric, the only unit in the game that can heal damaged units. An opposing school believes that putting a lot of high damage units up front to deal quick and devastating harm is a more effective strategy.
Since every setup type requires the opponent to use a different strategy to defeat, the type and pace of gameplay can vary widely. A game between two offensive setups will often be over in 15 minutes, while a game between two defensive setups might take hours.
Once the battle starts, the player who maneuvers his or her units best will generally have the advantage. After moving or attacking, every unit has a recovery time, during which they cannot make any action. The trick to good tactics is often to plan around these recovery times, moving units into advantageous positions while other units are recovering.
What makes TAO's tactical choices so fun, I think, is the distinctiveness of its units. Figuring how to use the specific units you have left to your best advantage in every situation can be tactically fascinating, provided you are playing someone of near-equal skill. If you have a scout, the unit with the longest range in the game, you can use it to snipe at your opponent's units while defending it with knights, slow melee units with very high defensive and offensive skills. The best target for such attacks are magic users, such as witches and pyromancers, who are vulnerable to scouts due to their low hit points. Magic users, in turn, have attacks which can hit multiple units at the same time, making them effective against a group of units bunched together. How you best use your units depends on the situation in the game.
There are two account types in TAO: The free "grey" account and the paid "gold" account, which costs $5 a month. Gold accounts have many more unit options than greys, making the gold game an entirely different experience than the grey. Gold accounts are also protected from the monthly server wipes which deletes all grey accounts. However, the grey game is very interesting on its own, so you can experience much of the fun of TAO without paying anything.
TAO can be played at http://www.tacticsarena.com/