Portals are Magical gateways between two places which may, or may not, even be in the same realm. Knowledge is limited but all known Portals have connections to Pagan Magic. They appear as Red Humming doors, mostly between two pillars, but have been known to be framed by other objects.
During The Dark Project, The Trickster used a Portal to connect to The Maw of Chaos and allow his army to invade The City, via the Hammerite Temple. There was another, greater portal, that was used by the Trickster to summon his Army into the maw, before he could dispatch them to the entrance Portal. The second Portal, had four coloured spheres imbrued with elemental forces. Garrett was intrusted to destroy those in order to stop the Army from appearing.
After the tricksters death and Garrett's escape, the Portal to the Hammerite Temple appeared to close. A new portal was created in the Cemetery linking to the Pagan village of Beck o' the Wills. Another Portal was created to link to the Maw, and had to be activate with two Gem Stones.