Sigh no Linux version, why? It's there on Steam. It's here with the Kingdom Rush games..

The aesthetic is absolutely a sci-fi Kingdom Rush. But when it comes to the gameplay, not so much. You basically play a tower offense. The enemy has bases which are defended by towers, and you must navigate your units and heroes, along the lanes to destroy these bases. Once you destroy one of the bases, you can build a base of your own, and additional units. The enemy also produces units that attack you using the same lanes.

It is a simplified RTS, and people hoping to play an RTS, ended up being disappointed. Personally, I've always liked upgrading the units/heroes, and that is still here.

My only complaint remains, that rogue likes and tower defense games need achievements, and this games lacks achievements on GOG. I'll buy it eventually, but as it is, I'm not too hyped about it.

This must have been changed in a later version, because the one I play now has an option of assigning keys to units.

It's a simplified RTS; more tactical than base building. Very enjoyable.
Do in-game achievements work? In this game they actually give you rewards (cash and tech points).

Don't know about that, since I don't own the game yet. I received a notification on STEAM when the game launched, and Ii checked some youtube let's plays. That's where I got most of the information I mentioned, that and steam reviews.

I like the art style, but sadly various reviews on Steam explain that the game is way too basic.

It's basic, but I'd argue about it being "too" basic. If you're looking for Warcaft/Starcraft/Red Alert, this is not what you're looking for. But it's a very fun game nonetheless.