Hi there! I understand that after upgrading the Mage Tower's cost reduction in Hero Defense King, it has become invisible and stopped working. I'd like to help you fix this.

First, can you tell me if this issue happens every time after you upgrade the tower, or did it happen just once? Also, does the tower remain invisible throughout the game level or only in certain parts? This info will help me guide you better.

Meanwhile, here's a step-by-step approach to troubleshoot this kind of issue on your Android 13 device with WebKit browser:

1. Restart the game: Close Hero Defense King completely and reopen it. Sometimes, the upgrade might not load properly the first time.

2. Clear cache: Go to your Android Settings > Apps > Hero Defense King > Storage > Clear Cache. This removes temporary files that might cause display bugs.

3. Check for game updates: Make sure you have the latest version installed, as bugs like this are often fixed in updates.

4. Reinstall the game: If the problem persists, uninstall Hero Defense King and reinstall it. Make sure your progress is saved via cloud save or connected account before uninstalling.

5. Test Tower without upgrade: If possible, try building a Mage Tower without upgrading to cost reduction to see if it's visible and functional. This can narrow down if upgrade triggers the issue.

6. Report bug: If none of the above work, this might be a bug specific to the tower upgrade effect. Please send a detailed bug report including your device info to the game support at [email protected] so developers can investigate further.

Let me know how it goes after trying these steps or if you have already done some of them! We'll get your Mage Tower back visible and working well. Are you able to test the tower without the upgrade and see if it shows up normally?