Board Thread:Lore Discussion/@comment-3348732-20130929164300/@comment-13446185-20130929234915

Martin didn't close off all portals to Oblivion he simply banished Mehrunes and restored the barriers between Mundus and Oblivion (which can be gotten around).

In the long-term he made it easier to reach Oblivion by destroying the Chim El-Adabal in-turn deactivating the White-Gold Tower, and making the barriers weaker than they were before the Oblivion Crisis.