While level rewards are random (with the exception of predefined drops of specific Cards), there seems to be only a certain number of possibilities just like in the Arenas. For example, let's take Serpent Land (after the 470 Crown Lock) and the Serpent Ruins as a smaller subset. I may very well prove my theory wrong in doing this, but perhaps it will be of some use - perhaps illuminating other patterns. Cells that are blank indicate a Gem (not a Card) reward.

Serpent Land (after 470 Crown Lock):

Serpent Ruins

Rewards in bolded text appear to be exceptions - they do not appear as possible rewards in the Serpentine Arena, at least not as those rewards currently stand. While possible Arena rewards did change in some of the game updates, I have no way of knowing for sure if all these Cards were among the changes made. I'm thinking not.

Shelly and Vermilion Helm are Rare Cards (which only appear once in my case), but the rest of the exceptions (Pirate Tower, Leather Cuirass, Lightning Tower, Healing Spring, Magic Buckler, Archer Tower, Freeze, Steel-Toe Boots, Slowdown Tower) are united in that they are all Common Cards. Is this significant? It would seem they all appear with some regularity. Girdle of Pain of course is an Epic Card, but it is predefined (not random) and cannot be anything else.

The Serpent Land (regular) levels seem to correlate better with my theory than the Serpent Ruins (tower) levels do. The only exception with Serpent Land rewards was the Leather Cuirass reward with "Magical Fields"  level. Do the King Tower levels follow a different set of rules than the regular levels?

As before, blank cells indicate a Gem reward. Question marks mean that I did not keep track of what Card I won. Bolded Cards do not appear as Orc Arena rewards.

Orc Land (Before Orc Tower)

Orc Tower (Up to Level 31)

You can see the tendency for rewards to be Orc Arena Cards is strong in the Orc Tower - the only exceptions being Summon Dragon and iRon, the latter of course being an Epic Card (with this being the predefined drop for a random Epic Card). On Level 28, I guess I didn't keep track of my 3rd Crown reward. In its place, UA141's Card for completing this level with 3 Crowns is shown in parentheses.