[RPG] When using the Wall of Fire spell, what shape is a ringed wall on a grid


I was looking at wall of fire and realized I wasn't sure how to represent the ringed version when on a grid. The spell states (emphasis mine):

You create a wall of fire on a solid surface within range. You can make the wall up to 60 feet long, 20 feet high, and 1 foot thick, or a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick […]

So assuming we maximize the diameter to 20 feet and use the following rule for areas of effect on a grid from the Dungeon Master's Guide (page 251), we get the following images with the second showing the outer squares, where the ring actually is:

Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal. If an area of effect is circular and covers at least half a square, it affects that square.

Ringed Wall of Fire
Ring Wall of Fire, Edges

However, this image seems incorrect; one could simply leave the ring by moving diagonally, avoiding taking damage entirely. Have I made a mistake in my drawings or is this just a case that the rule happens to not work well with?

Best Answer

You may want to use an alternative method from XGtE.

Your image is correct, and you cite all relevant rules. The Grid as a visual aid (DMG 250) is different from what the theatre of mind approach offers and provides different results, but there are other methods to adjudicate areas of effect on a Grid.

Xanathar's Guide to Everything's section about Spellcasting gives insight into this in its subsection about Areas of Effect on a Grid (XGtE 86-88). It offers two alternative options, the Template Method (XGtE 86-87) and the Token Method (XGtE 87-88) which offer different results, so you may choose one. From the Token Method:

Circles. [...] a circular area of effect becomes square in it, whether the area is a sphere, cylinder, or radius. For instance, the 10-foot radius of flame strike, which has a diameter of 20 feet, is expressed as a square that is 20 feet on a side, as shown in diagram 2.3.

You can find diagram 2.3. on page 86.

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