Whats the best way to spend gems?

Check out the answers already given here:

https://www.appgamer.com/rise-of-kingdoms/answers/12647-whats-the-best-thing-to-spend-gems

But basically it's a good idea to save them up for the More than Gems event, if you are planning to spend thousands. Going up the 7k or 12k gems, for example if you need to buy master blueprints-

Also look out for the events where you can use gems to unlock better pries in events, sometimes the prizes you get nearly equal the amount of gems it costs to make this upgrade.

Also look out for speedups discounted by 70% in the courier station, they are good value also.