Add the data model for structures and prevent them from being embedded in most actors.

module/documents/Item/Handler.mjs

@@ -1,11 +1,13 @@
 import AspectItem from "./Aspect.mjs";
 import SpellItem from "./Spell.mjs";
+import StructureItem from "./Structure.mjs";
 
 /** @extends {Item} */
 export class ItemHandler extends Item {
 	proxyTargets = {
 		aspect: AspectItem,
-		spell: SpellItem
+		spell: SpellItem,
+		structure: StructureItem,
 	};
 
 	constructor(data, ctx) {

module/documents/Item/Structure.mjs

@@ -0,0 +1,23 @@
+import { ItemHandler } from "./Handler.mjs";
+
+/** @this {ItemHandler} */
+async function _preCreate() {
+	if (this.isEmbedded) {
+		let actor = this.actor;
+		if (actor.type === "settlement") {
+			return await actor.preItemEmbed(this);
+		};
+		ui.notifications.error(
+			game.i18n.format(
+				`dotdungeon.notification.error.cant-embed-item`,
+				{ item: this, actor }
+			),
+			{ console: false, }
+		);
+		return false;
+	};
+};
+
+export default {
+	_preCreate,
+};

module/models/Item/Structure.mjs

@@ -0,0 +1,22 @@
+import { DescribedItemData } from "./DescribedItemData.mjs";
+
+export class StructureItemData extends DescribedItemData {
+	static defineSchema() {
+		const fields = foundry.data.fields;
+		return mergeObject(super.defineSchema(), {
+			one_night: new fields.NumberField({
+				initial: null,
+				nullable: true,
+			}),
+			upkeep: new fields.NumberField({
+				initial: null,
+				nullable: true,
+			}),
+			construction_length: new fields.NumberField({
+				min: 0,
+				initial: 0,
+				nullable: false,
+			}),
+		});
+	};
+};