I like the concept and I think a city being destroyed by a giant monster is a good fit for a grid based game. However, I think the game some times feels really hard, especially when multiple tanks show up or when the jet starts shooting missiles (although I think a game being punishing is very much in the spirit of a rogue like). The only thing I really would change beside difficulty is having a quicker way to heal life when there is no enemies around, like press space to skip forward 10 turns. Overall, fairly good for what I think is your first game.