Mini games/ Events are individual like the " Lucky little fin event", In this type of event one of the first buildings you are given for playing the game will be one that offers extra population plus some culture and extra goods.

If you place this building in your world you can then only upgrade it to the next stage by continuing to play the event & collecting the artifacts that will allow it to be upgraded.

Along with this building there are many other things ( buildings, army, goods, coins, supplies) to collect.

My thoughts 

IT'S OK

play it and see what you get !

For me space is always not enough for this type of building, The building they offer is OK but soon becomes outdated and you would be very lucky to get all the artifacts (10) needed to upgrade it to the max, so I almost always invariably keep it in stock and disenchant it for spell fragments.

For those just starting Elvenar it can be a real positive building to have, so put it on for a while until you can build better. 

IT'S GOOD

Usually through the event you get some random buildings. Choose carefully which ones you use as they will take up space, think space v reward. A good culture building is often won in these events.

Notes From Wiki Page:

Please note: All effects values depend on the chapter you are in at the time you add the building to your Inventory.

Expiring Buildings

Some event rewards have great power, but this power will only last a limited time. We call these types of rewards Expiring Buildings. When the boost of this building has expired, the building can remain in your city, still giving a little benefit.