A competitive, highscore-driven challenge. Players start with a small building area, something like 5x5x5. A time limit might be given (something around 3 minutes) in which the player can build a contraption. A side panel shows the next wave of attackers - waves can consist of foot soldiers, archers, stationary cannons and possibly more.

After build time is up, the wave starts and the player gains control. The sole goal is to survive the wave - either by defeating all enemies, or surviving for some set amount of time. If the core block doesn't get hit, the player gets an amount of points (maybe based on how intact the contraption still was), it gets fully repaired, the build area gets expanded a bit, and the player gets some time to upgrade their contraption again before the next wave starts.

Rinse and repeat until the core block gets destroyed. The total score gets calculated, displayed in an online ranking, and you can choose to try again from the start.

The waves can be hand-designed or procedurally created. There could even be a daily seeded challenge - same waves for everyone, only one try per day, with global highscores. It's thinkable to have different arenas with distinct settings and challenges later on (for example: basic one with forests and rocks, icy one with slippery ground, inside a volcano with incendiary obstacles and lava warriors, and so on).