Glad that you liked it and well-played!
Most of the visuals that are in the game are assets. I have only spent time coding the mechanics and the systems. I think it took me around 200-300 active hours developing the game, but there were many breaks in between. Those hours took place in 1 year.
OVERKILL is the project I learned how to properly program a game. It probably would take me less than 100 hours if I started coding it again from scratch with my current knowledge.
I hope that answers your question!