As is the case when I play Pokemon, my team focuses more on characters I like than on tactics and logistics, consisting at the moment of the 11th Doctor (in biker costume), Madame Vastra, Strax, Porridge, K9 Mark 2 and a UNIT Medic. Completing some levels may grant you a new character, or a new outfit to dress them up in. Or it may not; I must have played the Strax level getting on for fifty times before he randomly unlocked. There's currently an advent calendar promotion with daily freebies, and we have been promised a playable 7th Doctor before December is out.
You can throw money at the game if you wish - my flatmate paid for access to the fan area, allowing her to have Winston Churchill on her team - but you're not penalised for keeping it free. If you have too much leisure time and don't like sleeping, give it a download.