Another nicely-positioned realm would have been Summerdale, if it had Mt. Black Nastrond and Nifelhold to hold its eastern entrance and Valkyrja island on its west. Then there's the obvious Balance fortress, but I'm not sure it could keep a realm supplied with enough food to survive.
Regarding Asylon, it has a chain of regions that, if broken into, would tear the realm in two (Kybcyell, Uppervia, Itaufueld, Shuberstone), but it has a strategically-placed capital at Via. If fortifications were raised in Wallershire and Itaufield, the loss of Kybcyell corridor won't be as hampering (especially since we have food producers behind the capital, at Vakreno and the regions surround Koshtalom.