You can assign a task on multiple objects on the map. You need to tap on an object on the map, then tap on the icon in the bottom-left corner of the pop-up menu, and choose a task to assign.

To assign a task, please follow these steps:
1. Go to the resource tile/castle and tap on it;
2. In the bottom-left corner of the pop-up, a Map will appear;
3. Tap on the Map and you will have 1-2 options to assign.