Monuments and giants make a great back-up plan for defending vulnerable large (level 5+) cities. Since you can place a monument at any point before the game ends to get the points from it, hold them in reserve. Grow the city’s population just shy of what it needs to reach the next level. If that city is invaded, place a monument in its border and select super unit. The giant that spawns will kick out the invader. You must do this before the invader has a chance to convert!