Excellent.
Fun game.
Listen guys, don't concern yourself with too many other upgrades except sentry guns, initially. All of the weapons except the flamethrower are wildly ineffective, so save up for that. In earlier levels, you can typically get through two or three rounds without having to buy a gas fill-up. Fire the flamethrower in quick one fuel-unit bursts to conserve ammo, and you should be able to beat this shit no problem. Upgrade your wall periodically. Eventually, all you will need is, in fact, a sturdy wall and the flamethrower.