Ooo, more feedback, many thanks! Response in chronological order:
Crashes: Fuck, i thought i had eliminated those. Probably gonna have to fix that. :I
Zombie ships: Hee, you've just run into the apeshit coefficient. They're not invincible, just 5 times as tough (and valuable!) as their non-apeshit cousins. More specifically, by default, every enemy (EVERY.) have a 1 in 10000 chance of going apeshit, which, along with the aforementioned toughness boost, tends to make them move/fire a lot faster. You can turn it off if you wish; just open Config.ini with notepad and set Apeshit to 0. I made this feature precisely to elicit "wut" responses, although i can see that it could prove slightly inconvenient in some cases (i know my brother got surprised by an apeshit red sentry in stage 19, which amused me).
As for the limited lives thing, that's actually a very good point, i've been wondering how to ensure that people wouldn't be underpowered to deal with the end game. Biggest issue is probably that people could return to the shop at will during normal play, but i guess having spent lives NOT return due to this would mostly solve that issue (besides people who bail just before getting murderized). Might actually add this, although i'm not sure i could cram a "buy moar lief" button into the already crowded shop. We'll see. :V
And yeah, offense generally seems to be the more effective route, but that's how it almost always is IF you can avoid getting hit. I actually found that the final boss was easier with maxed defense than maxed offense, probably due to how powerful the final upgrade of the ladder is (+15% armour is a LOT when you already have 45%, along with resistance to defense-bypassing attacks); even if your DPS isn't as high, you can much more easily survive to deal out your DPS for a longer time. Case in point, if you have maxed defense and an active shield, you can survive a Despoiler shot on Nightmare, and since it makes Leviathan ram attacks scale with its health as opposed to an instant kill, it also makes it possible to come out on top in a collision if it's damaged enough. That besides, since you can take a bigger beating with defense, the danger bonus can get higher.
Regarding the sensor upgrade blips, they're just a couple of expanding circles blipping at the edge of the screen, so they're probably a bit too easy to miss (which i'll have to rectify). As for replacing the upgrade... Maybe? I kinda want there to be a price for playing fully offensively; you can make things dead good, but you have to compensate for the deficits yourself. The entire point of red wedges is to make the sides dangerous, because otherwise that's easily the safest place on the screen until leviathans show up. I suppose it is a bit unfair, limiting the playing field like that, but the alternative is that you can't shoot them before they make their move (which you can at the moment). I'll definitely consider it!