It may not have the razzle-dazzle of more recent tower defense games, but Sol Survivor is a solid entry in what is a crowded genre.
Cadenza Interactive does an excellent job in delivering a graphical style that is both elegant and visually appealing. I never felt like I wasn't able to identify what each turret's function was even in the thick of battle. I especially like that each type of turret damage is color coded, allowing you to easily spot where areas of your defense may be lacking in certain damage types. Piercing turrets are yellow, concussive turrets are orange, support fields are green and electrical damage is blue. It affords an interesting "temperature" to a fleshed out defense and makes each level a paint-by-numbers process of constructing creep murder.
The soundtrack is exceptional, and I must take a moment to point it out. It's not heartpounding, thrilling or action-y. You play the role of an ensign, rapidly rising through the ranks in a battle for humanity's survival. The music fits that theme. It's immersive, sad, and desperate. Kudos to the composer, because the soundtrack is amazing and fitting in every way.
Some tower defense fans will balk at the lack of "mazing" or "zoning". Each map already has its path laid out a la the original Warcraft tower defense maps. You simply place your turrets along the path and attempt to capitalize on the various types of damage you have available and the various creeps you are facing. As above, each tower can deal a certain type of damage. This damage is more or less effective against the various types of creeps, so planning your resource expenditure is necessary for a successful defense. Where Sol Survivor lacks the strategy of "zoning", it makes up for it in the addition of support powers. Each field commander brings a variety of support abilities to the table which can either be used to shore up a weakness in your tower selection or to bolster an already solid defense strategy. Your satellite arrays adapt to the support powers you use which allows them to level up over time, increasing their range of effectiveness, power and recharge time. The missions are carefully balanced to allow for this feature, making it necessary to take an active role in combat (rather than simply laying down towers and waiting for enough mass to upgrade or place more).
Sol Survivor also features multiplayer co-op, which is mind-blowing. I have yet to see another tower defense game offer up the same, and the fact that it has slipped under the radar is disappointing. You will be hard pressed to find partners (there are rarely any random people hosting an open lobby) but the playerbase is still active and you can find partners on the discussion hub. You can also send me an invite. I'm always willing to play.
All in all, Sol Survivor is an excellent game. It looks gorgeous, has an amazing soundtrack and it definitely has a story to tell. It may not have voice acting or other fancy features, but the gameplay is solid. I highly recommend it.