At least, that’s what its producer, Jeff Skalski, asserts:
Enjoying the iPad mini. Only complaint I have is its not a retina display, but man does Ultima Forever look fantastic on it. #UltimaForever
— Jeff Skalski (@Jeff_Skalski) April 3, 2013
This gives us some data on the performance requirements for Ultima Forever, since an iPad Mini is basically just a smaller iPad 2. The Mini uses the Apple A5 chipset, a PowerVR SGX543MP2 graphics subsystem, boasts 512 MB of RAM, and a 1024×768 display resolution. The iPhone 4S boasts roughly similar specifications, albeit with a slightly underclocked A5 and a somewhat lower display resolution. The current (fifth)-generation iPod Touch boasts nearly identical specifications to the 4S, albeit with the larger Retina display featured on the iPhone 5.
So, at a guess, it would appear that Ultima Forever will at least run on an iPad Mini/iPad 2, iPhone 4S, and fifth-generation iPod Touch. Whether older devices like the iPhone 4 are supported is…up for speculation.