If you or any other player has a card that can be played (except for the boot and wall card) then you should play it. Ideally, if between the players you have multiple cards that can be played, most of the time you should play the one closest to the castle but there are some situations where that is not the best idea.

It's okay to let the wall come down, in fact it helps you in a way because it takes out a monster without playing a card: for instance if you were in a game and you had a monster in the 8th and 9th cell and the monster behind them was in the 4th cell, and you had a red circle and a green square, it's better to play the red circle and let the wall come down and then you have time to prepare for the next monsters. If you however played the green square and by some unlucky chance came a runner. It would leave you with a monster in the 9th cell with no wall and no cards to remove it.

Remember there are 16 monsters you need to kill and there are four types. 10 normal monsters (including 3 starter monsters), 2 Runners, 2 Marchers and 2 Shovers. Keep in mind what might come next.

For the wall card only play it in two situations: 1. There is no other card that can be played 2. The wall needs to be put up because a monster that CANNOT BE REMOVED BY A CARD will walk into the castle or the next monster will move it or itself into the castle.

The boot card should only be played if you can't remove a monster that needs to be removed. Don't play them early unless you have to. DO NOT PLAY THE BOOT CARD ON A SPECIAL MONSTER.

If you want to make it easier or harder then note that the more players you have the easier it is to win (apart from solo)