Official EA description of this update.

larsyuipo wrote:
mmh, how will the upgrading characters work?? :hunf:

When the skin is being worn, click on the Castle (homer is on the initial, bart with the cardboard, Lisa with recycling one). All you have to do is click the gold button to upgrade the character. It does not TAKE your gold, you just need to have that amount acquired. So you can level all 3 at once to the highest you can.

battlefieldID194 wrote:
This really doesn't explain it. Can someone elaborate? Why am I seeing threads that people are suddenly needing to build houses to compete in the event? I don't have any house farms and have only 1 or 2 of each normal house (except brown houses). I just started building the castle and it wont be done until tonight. I have about $9.9M which I was saving for the sit-and-rotate. Should I hold off on buying that?

It depends how long you are out of your game. If you can't visit for 8 hours, your buildings will be destroyed and you miss out on gold or elixir since people could have visited you but had nothing to attack you. People are building houses, so they can get more visitors. More visitors = more gold and more elixir and more dice. So you help yourself get more personal prizes, have availability for neighbors to visit and randoms to visit. While I'm not a farmer, I started 12+ new houses that will be sold after the event ends. I want to make sure there's things to tap on in my town maximizing gold and elixir.