Town portal is available very early in the game. Always use "seer huts uncovered" mod, so you can find easily huts who reward you. After defeating Torosar (the first big battle), a hut north-west from Rome (at half day march) gives you a lot of spells, including town portal. Without it, you cannot finish the game. Of course, take that hut only when you got wisdom expert.

Seer huts uncovered