diff --git a/src/game.rs b/src/game.rs
index eeac51f..0fe0d2f 100644
--- a/src/game.rs
+++ b/src/game.rs
@@ -864,6 +864,10 @@ impl Game {
             self.gate_hp = (self.gate_hp + 8.0 * dt).min(GATE_HP);
         }
 
+        // every two seconds: corners between reinforced walls are closed
+        if (self.time / 2.0).floor() != ((self.time - dt) / 2.0).floor() {
+            self.reinforce_corners();
+        }
         self.update_contested();
         for i in 0..self.ents.len() {
             if !self.ents[i].alive || self.ents[i].held {
@@ -1673,6 +1677,37 @@ impl Game {
         }
     }
 
+    // An earth corner between two reinforced walls (and diagonal to own floor) cannot be
+    // reached by the imps: it is reinforced together with the walls. The floor condition keeps
+    // this from spreading into the earth behind the walls.
+    pub fn reinforce_corners(&mut self) {
+        let (w, h) = (mw() as i32, mh() as i32);
+        let wall = |m: &Map, x: i32, y: i32| m.get(x, y).map_or(false, |n| n.kind == Kind::Earth && n.reinforced && n.owner != Owner::Hero);
+        let floor = |m: &Map, x: i32, y: i32| m.get(x, y).map_or(false, |n| n.owner == Owner::Player && n.walkable());
+        let mut add = Vec::new();
+        for y in 1..h - 1 {
+            for x in 1..w - 1 {
+                let t = &self.map.t[idx(x, y)];
+                if t.kind != Kind::Earth || t.reinforced || t.marked || t.owner == Owner::Hero {
+                    continue;
+                }
+                for (ax, by) in [(1, 1), (1, -1), (-1, 1), (-1, -1)] {
+                    if wall(&self.map, x + ax, y) && wall(&self.map, x, y + by) && floor(&self.map, x + ax, y + by) {
+                        add.push(idx(x, y));
+                        break;
+                    }
+                }
+            }
+        }
+        for i in add {
+            let t = &mut self.map.t[i];
+            t.reinforced = true;
+            t.hp = t.hp.max(12.0);
+            let (x, y) = xy(i);
+            self.fx(8, x as f32 + 0.5, y as f32 + 0.5, 0.0, 0.0, 0.5, 0xff4040e0);
+        }
+    }
+
     // tiles within two of a fighting hero
     fn update_contested(&mut self) {
         let n = self.map.t.len();
@@ -4178,6 +4213,24 @@ mod tests {
         assert!(ok * 10 >= samples * 8);
     }
 
+    #[test]
+    fn wall_corners_are_reinforced_too() {
+        let mut g = Game::new(3, 64, 64);
+        let (cx, cy) = g.map.core;
+        // the start area spans cx-3..cx+5 / cy-3..cy+5: its walls are at cx-4 and cy-4
+        let (x0, y0) = (cx - 4, cy - 4);
+        for k in 1..4 {
+            g.map.t[idx(x0 + k, y0)].reinforced = true;
+            g.map.t[idx(x0, y0 + k)].reinforced = true;
+        }
+        assert!(g.map.t[idx(x0, y0)].kind == Kind::Earth && !g.map.t[idx(x0, y0)].reinforced);
+        g.reinforce_corners();
+        assert!(g.map.t[idx(x0, y0)].reinforced, "the corner is reinforced");
+        // nothing spreads into the earth behind the walls
+        g.reinforce_corners();
+        assert!(!g.map.t[idx(x0 - 1, y0)].reinforced && !g.map.t[idx(x0, y0 - 1)].reinforced && !g.map.t[idx(x0 - 1, y0 - 1)].reinforced);
+    }
+
     #[test]
     fn imps_reinforce_walls() {
         let mut g = Game::new(3, 64, 64);