Similar to another concept I've seen but this once suffers from a simple dominant strategy - get one white tower then just connect tower - tower to have white towers only. Concept could be improved by removing this dominant strategy if you had more time. Maybe having colors be different towers that are all needed or need to work together, something simple like: red (lvl 1) = continuous beam attack (weak/fast); blue (lvl 1) = ice/slow; green (lvl1) = weak AoE; purple (lvl 2) = magic dmg (better against armored mobs); light blue (lvl 2) = chain lightning tower; yellow (lvl 2) = contant AoE dmg; white (lvl 3) = buffs surrounding towers. For added depth, you could make it so colored beams cannot cross on the map so tower placement and links are another strategy issue as well