Protector IV: Mercenary
Almost everything is done with the mouse, with mouse+[shift] to place multiples of the same unit, and [spacebar] to cancel out of a unit placement. The main part of the game is tower defense. Scouting reports about upcoming waves appear in the lower left, with important information such as the elemental strengths and weaknesses and other vulnerabilities. Based on this information, strategically place units on the board. Once you start a wave, killing enemies will increase your money and give your units experience. From there, you can choose to place more units and level up the units you have.
Between quests, you hire and equip heroes. "Heroes" are associated with two units and one element, and you can take three heroes into a quest. Depending on the quest, you might try to have all one element and six different units, or the same units but three different elements, or something different. In whatever case, you'll clearly want a full arsenal of heroes at your disposal. In addition to a main storyline of quests with linear progression, the game is full of side quests that pop up randomly depending on your skill level and your relationship with the eleven different factions. The side quests are offered by the different factions, and only by increasing your favor with different factions will you be able to hire the very strongest heroes. There's also an option to fund expeditions to discover rare artifacts, whose name, "Protectomon", suggests a certain "gotta catch 'em all" factor.
The inability to save during a quest will probably cause dinnertime and bedtime brawls between parents and children across the world. Parents: don't forget to tell your kid that there is no penalty for ending or losing a quest early. On the contrary, you get to keep any loot you've found up to that point, and you also get some experience and money based on your score.
The only real fault I can find with the game is that it's still using the same (terrible) sound effects. The shrieking bat in particular, when you're fighting a large, fast wave, is ear-bleedingly annoying. Thankfully, nothing in gameplay really depends on sound effects, so you can turn them off by clicking on the volume icon at the top of the screen and selecting the sound effects. If you hated the previous games, there's not going to be anything in Protector IV to make you change your mind. For the large number of established fans, it's what you loved, only bigger and fresher. Hit mute and have fun!