"You don't want to leave it un-defended"
But is the enemy you are attacking your only enemy?
Looks like I can`t get away without giving away some further details about our MMORTS model:
As I mentioned before we are sacrificing world geometry for the sake of gameplay - which mostly shows in time it takes to travel in our gameworld from point A to point B. Couple of arguments in defense of our approach:
-Time is really relative in RTS: it takes longer for units to walk across town as it does to construct a town wall.
-These days instant teleporting around the gameworld becomes key feature - Oblivion for example.
Now to get back to gameplay and your question,
-there is a way to easily switch game view between location - like your town, or enemy town in real-time, without even 1 second delay. Think about the way you use mini-map to jump from one location to another in typical RTS. It`s completely intuitive.
-there is a way to move units in real-time (with a 30-60 second delay) from your town to enemy town
The ‘victim’ whose castle you plan to besiege, can attack you right back. And to lead on to what Doug hinted on, don’t forget your allies, and your enemies allies.