How do I raise a town defence shield?

Shields can be used by tapping on your Castle building (in your city) or on the icon for your city on the map and then select the Town buffs option. This will show a list of all the buffs you can activate. Tap on the Shield option and then select the duration of shield you want to start.

If you don't have a shield item then you will need to buy it with gems. You can also buy some shield items from the Alliance store with alliance contribution points. Then activate them via the town buffs screen.