diff --git a/js/main.js b/js/main.js
index 4ee836e..714fc4d 100644
--- a/js/main.js
+++ b/js/main.js
@@ -67,6 +67,33 @@ async function fetch(url) {
 
 const PAGE_TITLE = "Lexy's Labyrinth";
 
+const DIRECTIONS = {
+    north: {
+        movement: [0, -1],
+        left: 'west',
+        right: 'east',
+        opposite: 'south',
+    },
+    south: {
+        movement: [0, 1],
+        left: 'east',
+        right: 'west',
+        opposite: 'north',
+    },
+    west: {
+        movement: [-1, 0],
+        left: 'south',
+        right: 'north',
+        opposite: 'east',
+    },
+    east: {
+        movement: [1, 0],
+        left: 'north',
+        right: 'south',
+        opposite: 'west',
+    },
+};
+
 class Tile {
     constructor(type, x, y, direction = 'south') {
         this.type = type;
@@ -91,6 +118,24 @@ class Tile {
         return tile;
     }
 
+    blocks(other, direction) {
+        if (this.type.blocks)
+            return true;
+
+        if (this.type.thin_walls &&
+            this.type.thin_walls.has(DIRECTIONS[direction].opposite))
+            return true;
+
+        if (other.type.is_player && this.type.blocks_players)
+            return true;
+        if (other.type.is_monster && this.type.blocks_monsters)
+            return true;
+        if (other.type.is_block && this.type.blocks_blocks)
+            return true;
+
+        return false;
+    }
+
     ignores(name) {
         if (this.type.ignores && this.type.ignores.has(name))
             return true;
@@ -179,33 +224,6 @@ class Cell extends Array {
     }
 }
 
-const DIRECTIONS = {
-    north: {
-        movement: [0, -1],
-        left: 'west',
-        right: 'east',
-        opposite: 'south',
-    },
-    south: {
-        movement: [0, 1],
-        left: 'east',
-        right: 'west',
-        opposite: 'north',
-    },
-    west: {
-        movement: [-1, 0],
-        left: 'south',
-        right: 'north',
-        opposite: 'east',
-    },
-    east: {
-        movement: [1, 0],
-        left: 'north',
-        right: 'south',
-        opposite: 'west',
-    },
-};
-
 class Level {
     constructor(stored_level) {
         this.stored_level = stored_level;
@@ -387,10 +405,7 @@ class Level {
             if (! blocked) {
                 let goal_cell = this.cells[goal_y][goal_x];
                 goal_cell.each(tile => {
-                    if (! (
-                        tile.type.blocks ||
-                        (tile.type.thin_walls && tile.type.thin_walls.has(DIRECTIONS[direction].opposite))
-                    ))
+                    if (! tile.blocks(actor, direction))
                         return;
 
                     if (actor.type.pushes && actor.type.pushes[tile.type.name]) {
@@ -400,7 +415,7 @@ class Level {
                     }
                     if (tile.type.on_bump) {
                         tile.type.on_bump(tile, this, actor);
-                        if (! tile.type.blocks)
+                        if (! tile.blocks(actor, direction))
                             // It became something non-blocking!
                             return;
                     }
diff --git a/js/tiletypes.js b/js/tiletypes.js
index b927931..b651865 100644
--- a/js/tiletypes.js
+++ b/js/tiletypes.js
@@ -100,12 +100,15 @@ const TILE_TYPES = {
 
     // Terrain
     dirt: {
-        // TODO block monsters, and melinda only without the hiking boots
+        blocks_monsters: true,
+        blocks_blocks: true,
+        // TODO block melinda only without the hiking boots; can't use ignore because then she wouldn't step on it  :S  also ignore doesn't apply to blocks anyway.
         on_arrive(me, level, other) {
             me.become('floor');
         }
     },
     gravel: {
+        blocks_monsters: true,
     },
 
     // Hazards
@@ -249,39 +252,48 @@ const TILE_TYPES = {
     dirt_block: {
         blocks: true,
         is_object: true,
+        is_block: true,
     },
 
     // Critters
     bug: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
         movement_mode: 'follow-left',
     },
     paramecium: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
+        movement_mode: 'follow-right',
     },
     ball: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
     },
     blob: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
     },
     teeth: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
     },
     fireball: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
         movement_mode: 'turn-right',
         ignores: new Set(['fire']),
     },
     glider: {
         is_actor: true,
         is_object: true,
+        is_monster: true,
         movement_mode: 'turn-left',
         ignores: new Set(['water']),
     },
@@ -359,6 +371,8 @@ const TILE_TYPES = {
         is_object: true,
         is_chip: true,
         is_required_chip: true,
+        blocks_monsters: true,
+        blocks_blocks: true,
         on_arrive(me, level, other) {
             if (other.type.is_player) {
                 level.collect_chip();