Soulcaster

Soulcaster and its sequel Soulcaster II are a pair of retro-style games for Xbox Live Arcade and PC that blend classic Gauntlet (1985 video game)-style top-down Action Adventure games with Tower Defense.

The player character is totally helpless on his own. However, he has the ability to summon avatars of powerful warriors to defend him:

Shaedu, the archer, who can attack enemies at range with her bow as long as she has a line of sight.
Aeox, the knight, who has the strongest defense but can only attack adjacent enemies.
Bloodfire, the alchemist, whose bombs deal Area of Effect damage and can be thrown over walls, but has very low defense and explodes when killed.

With the help of these three allies--as well as the mysterious Merchant, who provides RPG Elements in the form of buyable upgrades--your task is to defeat the waves of monsters that attack you as you travel through a series of dungeons.

The official website is here.

Tropes used in Soulcaster include:

Cycle of Hurting: Happens if you get surrounded and you have no scrolls left.
Event Flag: The trigger is usually standing on a certain square.
Fighter, Mage, Thief: Aeox, Bloodfire, and Shaedu, respectively.
Floating Continent: One of the levels in Soulcaster II is a flying city.
Game Level
Genre Buster: Elements of a Tower Defense while playing nothing like a normal Tower Defense; elements of a top-down Action Game while playing nothing like a normal top-down Action Game.
Inescapable Ambush: Ubiquitous. Basically happens in every level--the way back is blocked, and the only way to progress is to kill all the enemies.
Mission Pack Sequel: The gameplay of Soulcaster II is identical to the original; mostly, it's just new levels.
Night of the Living Mooks: Zombies are one of the main types of mooks you'll encounter.
Password Save: Used in the Xbox version of the first game.
Retraux: In the style of the 8-bit era.
Robe and Wizard Hat: Worn by the title character.
RPG Elements: Visit the Merchant to upgrade your warriors' range, power, attack speed, and HP.
Sequel Hook: At the very end of the second game, your companions ask you to help them in another quest.
Shout-Out: The famous "JUSTIN BAILEY" password from the original Metroid works in the first Soulcaster. It starts you off partway through the game with a bunch of upgrades on all three souls.
Smart Bomb: Scrolls deal massive damage to all nearby enemies. They're a good panic button in case you get surrounded.
Summon Magic: The premise of gameplay. You can summon the three souls at will.
The Turret Master: Since your allies are stationary, this is very much at play.