It's the same sort of mechanism- Risk-style campaign map combined with RTS style battles. From memory, the controls for the latter games are pretty similar to what Shogun's were as well. You can auto-resolve the battles if you're not an armchair general type, but the battles are such good fun, you'd be mad to.
If you find it a bit complex, the easiest way to think of it is as a tarted-up scissors-paper-stone game. For example, cavalry decimate archers, archers pick off pikemen, pikemen fuck cavalry. There's more refinements obviously, like height (higher up is better generally), morale, flanking, fatigue etc etc, but at its most fundamental this is basically what it is.
You can get the original Medieval for a fiver now, or Rome for under a tenner and they're both excellent, so it might be worth getting one of them if you don't want to splurge £25 on a new game. Rome is closest in terms of gameplay and graphics, Medieval obviously in terms of period, units, influence of religion etc.
