What about both players attacking and defensing at the same time? Both castles could have a way in and a way out (2 one-way paths, or simpler would be a single path both players use but it might just end up with both players spamming units which kill each other 1 at a time at the middle...)

There's a bunch of tower defense maps on Starcraft 2 which are very similar to what you describe. You should check them out, they may already be doing what you have in mind.

No offence but I had no idea what is "tower defence" before coming to gamedev and after learning I can't imagine a worse way of 'strategy' gaming.

My feeling is that many games have shown that many players aren't fans of the "plan it then watch it unfold" style of play. Most players want all phases to be interactive. So perhaps you do all the planning etc, you start the attack... and then you have an arcade or RTS control system to help your guys win. I'd suggest you only control the defenses and you hope you can hold out until your attackers beat the opponent. But then that might sound close to Orcs Must Die.

I think what makes Orcs Must Die work well is that you can run around shooting stuff while your traps do their thing, so you don't feel like you're a spectator.

I think it would be cool if one player went, chose his move, the next player went, chose his move, then the outcome was shown in a single scene, and if they clashed, a mini war would start, turn based