Part of why shoot ’em ups are great is explosions. Massive fancy explosions everywhere. Kerblam! Foosh! Splode! Tower defence games are also great at spewing fire, debris, and particles: that’s how you know you’ve defended that thing real good. Sprang! Clonk! Fsssh! Combine the two and you’d surely have fire, lasers, missiles, and explosions just everywhere. Looking at Excubitor shows yep, that’s about right. The shmup/TD has come to my attention by popping up on Steam Greenlight, but it’s got a public alpha version we can play right now too. Blammo!