Your observations are correct. The original idea was to have an explosion take one life, and touching zombies be instant death. But it was near impossible, and with limited development time, I opted for quick fixes. The first was to have health hit points starting at 90. The second which worked less well was to slowly increase the number of zombies based on the number of walls left, but I didn't have time to balance it, and at the moment, you do get overwhelmed.
Thank you for your feedback.