A brief look on your design clarifies, that it can't be fully routed on a single layer with the present technology (trace width and spacing). Furthermore I notice, that it has exactly one bypass capacitor (near the voltage regulator) and none at other chips. So there may be serious doubts, if the circuit is working reliably this way.
There's a certain chance, that reassigning the processor pins according to routing requirements, as mentioned by Keith, may help. But you would badly want a better ground net wiring and supply bypassing.