Beam should be incompatible with Ping tower, I believe it's a bug.
Decoder tower is overpowered, add linker, fork and splash and the game becomes easy - slow makes enemies clump very fast and densely for splash to do wonders, fork + link effectively quadriples damage output. You can beat any level with just this tower.
Ping tower is slighly overpowered, only because you can't add certain mods to it and it has just 2 slots.
Nice setting and really well thought level design/dificulty curve, I liked the game and surely would recommend it to anyone that likes TD.