Such a great idea, I had a lot of fun playing this, thanks for making it! :D
There's probably an interesting avenue you can explore where rather than "firing" firewalls up the screen, you drag and drop them - so that you can strategically try to maintain a certain "structure". (I suppose you could explore adjacency bonuses, upgrades, and varying "attack types" down that road!) but I don't know if that fits your vision!
I don't know if I am rubbish, or have terrible luck, but I found the game had two distinct difficulty phases: - It's super hard, I never survive long enough to buy a single server (also there may be a bug where - if you don't refresh your browser - the difficulty doesn't fully reset!) - Or, once you've bought a single extra server, you can suddenly almost hold down space and never interact again (except to fill the screen with firewalls occasionally)
What I found was that if you try to place any defences during the first ~minute, it feels like you always end up in a war of attrition that cannot be won. So the winning move seems to be to do nothing for ~a minute, race to buy a server, and then you're golden. It took me ages to discover this, though!
This leads to an interesting second point, which is that until you manage to survive long enough to buy a second server, you don't get an opportunity to learn that the server count goes down when overloaded, acting as a health bar. Perhaps the player should start with >1 server so they can learn this mechanic?
A minor visual improvement: I found that the money going up in increments felt really jarring, I don't know if I'm just weird, but I think if it updated more like 2-4 times per second, it'd feel more responsive?
You might want to consider making the server cost start cheaper, and become more expensive with each purchase?
Anywho, I hope all of that feels like useful feedback, please don't take it negatively - I only have this feedback for you because the idea was so compelling I _had_ to play it enough to generate these opinions! :D