I've made in-app purchases in TowerMadness 2 and do not see them anymore due to deleting/re-installing the app or switching devices.
Please make sure you are signed into the same Apple ID or Play Store ID that you original made your purchase through.
1) Launch TowerMadness 2.
2) In the main menu, tap the "gear" icon in the lower left corner to access the settings app, and tap "Restore Purchases". You will see your purchases restored when you enter Xen's Shop.
Please note: In-app purchases will not transfer to different account IDs. This is a restriction set by the App Store and Google Play.