diff --git a/server/config.ts b/server/config.ts
index d494df5..72da832 100644
--- a/server/config.ts
+++ b/server/config.ts
@@ -3,7 +3,15 @@ import commandLineUsage from "command-line-usage";
 import fs from "graceful-fs";
 import url from "url";
 import { parse } from "sparkson";
-import { Config, hasOwnProperty, Url, Version } from "./types";
+import {
+    Config,
+    hasOwnProperty,
+    isLayerConfig,
+    isPlainObject,
+    isString,
+    Url,
+    Version,
+} from "./types";
 
 export let config: Config = {} as Config;
 export let version: Version = "unknown" as Version;
@@ -89,6 +97,24 @@ export function parseCommandLine(): void {
 
     config = parse(Config, configJSON);
 
+    if (!isPlainObject(config.client.coordsSelector.layers)) {
+        console.error(
+            "Error in config.json: client.coordsSelector.layers is not an JSON object."
+        );
+        process.exit(1);
+    }
+
+    for (const [id, layerConfig] of Object.entries(
+        config.client.coordsSelector.layers
+    )) {
+        if (!isLayerConfig(layerConfig)) {
+            console.error(
+                `Error in config.json: client.coordsSelector.layers[${id}] is not a valid layer config.`
+            );
+            process.exit(1);
+        }
+    }
+
     function stripTrailingSlash(url: Url): Url {
         return url.endsWith("/")
             ? (url.substring(0, url.length - 1) as Url)
diff --git a/server/shared/types/index.ts b/server/shared/types/index.ts
index 0421aad..5c8a77e 100644
--- a/server/shared/types/index.ts
+++ b/server/shared/types/index.ts
@@ -1,4 +1,9 @@
-import { ArrayField, Field, RawJsonField } from "sparkson";
+import {
+    ArrayField,
+    Field,
+    RawJsonField,
+    registerStringMapper,
+} from "sparkson";
 
 // Types shared with the client.
 export type TypeGuard<T> = (arg: unknown) => arg is T;
@@ -282,12 +287,50 @@ export function isCoordinatesConfig(arg: unknown): arg is CoordinatesConfig {
     return isNumber(coords.lat) && isNumber(coords.lng);
 }
 
+export type LayerOptions = {
+    attribution: string;
+    subdomains?: string;
+    maxZoom: number;
+};
+
+export function isLayerOptions(arg: unknown): arg is LayerOptions {
+    if (!isPlainObject(arg)) {
+        return false;
+    }
+    const obj = arg as LayerOptions;
+    return (
+        isString(obj.attribution) &&
+        isOptional(obj.subdomains, isString) &&
+        isNumber(obj.maxZoom)
+    );
+}
+
+export type LayerConfig = {
+    name: string;
+    url: Url;
+    type: string;
+    layerOptions: LayerOptions;
+};
+
+export function isLayerConfig(arg: unknown): arg is LayerConfig {
+    if (!isPlainObject(arg)) {
+        return false;
+    }
+    const obj = arg as LayerConfig;
+    return (
+        isString(obj.name) &&
+        isUrl(obj.url) &&
+        isString(obj.type) &&
+        isLayerOptions(obj.layerOptions)
+    );
+}
+
 export class CoordinatesSelectorConfig {
     constructor(
         @Field("lat") public lat: number,
         @Field("lng") public lng: number,
         @Field("defaultZoom") public defaultZoom: number,
-        @RawJsonField("layers") public layers: JSONObject
+        @RawJsonField("layers") public layers: Record<string, LayerConfig>
     ) {}
 }