Hi there! I understand that after upgrading your mage tower to reduce its cost in Hero Defense King, it became invisible and stopped working. That's definitely a tricky issue. Let’s figure this out together.

Can you tell me a bit more about what happened just before the tower turned invisible? For example:
- Did the tower vanish immediately after the upgrade, or after restarting the game?
- Have you tried restarting your device or the game since the upgrade?
- Are other towers working fine?
- Is the game fully updated to the latest version?

In the meanwhile, here are some steps you can try:

1. Restart the Game: Close Hero Defense King completely and restart it. Sometimes the upgrade visuals don't load properly until the game restarts.
2. Clear Cache: On your Android 13 device, go to Settings > Apps > Hero Defense King > Storage > Clear Cache. This can fix visual glitches without affecting your saved data.
3. Check for Updates: Make sure the game is updated to the latest version from your app store. If this is a known bug, the latest patch might fix it.
4. Revert Upgrade (if possible): Check if the game allows you to reset tower upgrades or re-purchase the upgrade to see if it restores functionality.
5. Check Permissions: Sometimes, visual issues occur if the app doesn’t have necessary permissions. Ensure the app has storage access in your Android settings.

If none of these solve it, the issue might be a game bug related to that specific upgrade path. In that case, contacting the game support team directly through their official channels may be necessary — for example, via [email protected].

Please try these and let me know what changes, or if you want me to guide you through any step!