AWESOME! BEST OF ALL ZOMBIE GAMES!!!
About the advice for getting past level 19:
the trick is to use time wisly. spend an even amount of hours searching for survivers, (an extra gunman could save your life when the horde gets to a huge size)
and search for weapons as just as well (Chainsaw good for close combat as secondary weapon, while your primary would be a gun of your choice)
Importantly, you must keep track of the status of your baracade. If the wall comes down, you're as good as dead.
About combat: always shoot the zombie closest to you (If one is running, by all means, despose of him first) If ANY reach the wall, Kill them, they are a major problem, even worse if they carry weapons.
Lastly, Move around. Your team is not exactly a bunch of war veterans. their combat skill sucks! you need to move about and be aware of every zombie.
Hope this helps!