So I played some
OTTTD (Over the Top Tower Defense) this afternoon, as it's currently part of the
Humble Weekly bundle in the $1 tier.
The game is... not very good. It's kind of like a mix of traditional tower defense and XCOM, but taking the worst of both genres. You can get up to three "heroes" on the battlefield which can repair your towers and do additional damage, but the heroes suffer from a 1 hour respawn time when killed - unless you revive them using the in-game currency. It's pretty obvious given the different types of in-game currency that this game was originally designed with in-app purchases in mind. Blech!
As for the actual tower defense side of the game, it's pretty terrible. Even though you can level up your towers in multiple ways, it hardly ever feels like you're able to do more than just a smidgen of damage to the incoming attackers - you never have that "hahaha I'm invincible!" feeling that I really love in a good tower defense game. In addition to that, the difficulty spikes on each level are massive - meaning you'll be playing and replaying the same levels over and over again trying to get a higher score or grind up enough gold to buy better weapons or armor for your heroes.
The only positive I could find was that the achievements seemed to be pretty easy to get, especially at first. I unlocked 27% in the first hour, and they seem to have
a pretty good rate of progression throughout. But I'm not sure who I would recommend this game to, certainly not fans of tower defense when there are so many other good offerings on PC. I'm just glad I only paid $1 for it.