It started with Kastle 2, Bastl Instruments’ cute and clever handheld. It could be a multi-effect, a sample player, a synth. Then Citadel modules came along and did those things in Eurorack. Now, Bastl’s got a surprise: you can buy these as DIY kits, and a big chunk of the Raspberry Pi-powered platforms are being released as open source. Here are all the details.