As soon as you can, invest gold in the Museum . This provides a passive "daily" income based on your progress, which helps fund expensive late-game gear.
In the original game, you must reach Wave 25 in Survival Mode to unlock the ultimate arena boss battle.
Upgrading your training facilities enough times will unlock secret decorations like the toy whale and a cat in the window .
There are 13 hidden children scattered across various game screens. Finding them all provides rewards and is required for completion.