Any level is viable in Cyrodiil once you enter it. All mobs are level 50, you're normalized to level 50. Lacking skills and abilities at 10, so you'll have a harder time fighting... but you can get to Cloud Ruler temple if you managed the walk.

You need to get out of Cyrodiil and back to your home zone first.

The 1-50 leveling sequence will take you through the zones for your alliance. Once you hit level 50 and complete the main story, you will get to explore the other alliance's zones and do their quests. By the time you hit VR 10 you should have the ability to travel anywhere.

There is no Imperial City currently. At level 10 you can explore all of Cyrodiil if you can survive the PvP. You can also explore your entire alliance zone, what's in the game, at sub 10 level as long as you know the gate/boat locations, how to sneak, and have a few filled soul gems just in case. Now if you plan to do quests, the minimum level will depend in part on how many player are around, and in part on your personal skill. To explore other alliances on the same character, you will need to be lvl 50 and to have completed the Main quest.