diff --git a/core/include/vf/Dimension.h b/core/include/vf/Dimension.h
new file mode 100644
index 0000000..908b2c6
--- /dev/null
+++ b/core/include/vf/Dimension.h
@@ -0,0 +1,156 @@
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <unordered_set>
+#include <vector>
+
+#include "EndGen.h"
+#include "NetherGen.h"
+#include "World.h"
+#include "WorldGen.h"
+
+namespace vf {
+
+enum class DimensionId : uint8_t {
+    Overworld = 0,
+    Nether = 1,
+    End = 2,
+    Count,
+};
+
+// Per-dimension rules. coordinateScale counts overworld blocks per one block
+// of this dimension, so the Nether's 8.0 is the familiar 1:8 ratio.
+struct DimensionConfig {
+    DimensionId id = DimensionId::Overworld;
+    int floorY = 0;
+    int ceilingY = 128;
+    bool hasSky = true;
+    bool hasCeiling = false;
+    bool hasDayCycle = true;
+    bool waterEvaporates = false;
+    bool bedsExplode = false;
+    float coordinateScale = 1.0f;
+    float ambientLight = 0.0f;
+    uint32_t seedSalt = 0u;
+
+    static DimensionConfig forDimension(DimensionId id);
+    static const char* nameOf(DimensionId id);
+
+    int height() const { return ceilingY - floorY; }
+    int lowestPlaceableY() const { return floorY + 1; }
+    int highestPlaceableY() const { return hasCeiling ? ceilingY - 3 : ceilingY - 2; }
+    bool containsY(int y) const { return y >= floorY && y < ceilingY; }
+    int clampY(int y) const;
+};
+
+// One world plus the generator that fills it. Exactly one generator is live
+// per dimension; the others stay null.
+class Dimension {
+public:
+    Dimension(const DimensionConfig& dimensionConfig, uint32_t worldSeed);
+
+    const DimensionConfig& config() const { return cfg; }
+    DimensionId id() const { return cfg.id; }
+    uint32_t seed() const { return derivedSeed; }
+
+    World& world() { return blocks; }
+    const World& world() const { return blocks; }
+
+    bool columnGenerated(int columnX, int columnZ) const;
+    void generateColumn(int columnX, int columnZ);
+    int generateArea(int columnX, int columnZ, int columnRadius);
+    std::size_t generatedColumnCount() const { return columns.size(); }
+
+    void tick(int steps) { elapsed += steps; }
+    long long ticks() const { return elapsed; }
+    // Local time of day in [0, 1). Dimensions without a day cycle are frozen
+    // at their ambient level and always report the same fraction.
+    float dayFraction() const;
+
+    int highestSolidY(int gx, int gz) const;
+    // Lowest feet position at or above fromY that rests on something solid and
+    // has `clearance` free blocks above it.
+    bool standingSpot(int gx, int gz, int fromY, int clearance, int& outFeetY) const;
+
+    const WorldGen* overworldGenerator() const { return overworld.get(); }
+    const NetherGenerator* netherGenerator() const { return nether.get(); }
+    const EndGenerator* endGenerator() const { return end.get(); }
+
+private:
+    DimensionConfig cfg;
+    uint32_t derivedSeed;
+    World blocks;
+    long long elapsed = 0;
+    std::unordered_set<Vec3i, Vec3iHash> columns;
+    std::unique_ptr<WorldGen> overworld;
+    std::unique_ptr<NetherGenerator> nether;
+    std::unique_ptr<EndGenerator> end;
+};
+
+// Owns one Dimension per DimensionId; each ticks and generates independently.
+class DimensionSet {
+public:
+    static constexpr int dimensionCount = static_cast<int>(DimensionId::Count);
+
+    explicit DimensionSet(uint32_t worldSeed = 1337u);
+
+    uint32_t seed() const { return baseSeed; }
+
+    Dimension& dimension(DimensionId id) { return *worlds[static_cast<std::size_t>(id)]; }
+    const Dimension& dimension(DimensionId id) const { return *worlds[static_cast<std::size_t>(id)]; }
+
+    Dimension& overworld() { return dimension(DimensionId::Overworld); }
+    Dimension& nether() { return dimension(DimensionId::Nether); }
+    Dimension& end() { return dimension(DimensionId::End); }
+
+    void tickAll(int steps);
+    void tickOnly(DimensionId id, int steps) { dimension(id).tick(steps); }
+    std::size_t totalChunks() const;
+
+private:
+    uint32_t baseSeed;
+    std::vector<std::unique_ptr<Dimension>> worlds;
+};
+
+struct TravelResult {
+    DimensionId from = DimensionId::Overworld;
+    DimensionId to = DimensionId::Nether;
+    Vec3i source;
+    Vec3i scaledTarget;              // pure coordinate mapping
+    Vec3i target;                    // scaledTarget clamped into the destination
+    bool clampedToFloor = false;
+    bool clampedToCeiling = false;
+    bool needsPortal = true;
+    int searchRadius = 0;
+    int nearestIndex = -1;           // index into the candidate list passed to plan()
+    int nearestDistanceSquared = 0;
+};
+
+// Moves a position between dimensions: scales the horizontal coordinates by
+// the ratio of the two coordinate scales, clamps the result into the target's
+// vertical bounds, and reports whether an existing portal was close enough to
+// reuse or a new one has to be raised.
+class DimensionTravel {
+public:
+    explicit DimensionTravel(DimensionSet& set) : worlds(set) {}
+
+    DimensionSet& dimensions() const { return worlds; }
+
+    static Vec3i scale(const Vec3i& pos, const DimensionConfig& from, const DimensionConfig& to);
+    static int searchRadiusFor(DimensionId to);
+    static int buildRadiusFor(DimensionId to);
+
+    TravelResult plan(DimensionId from, DimensionId to, const Vec3i& pos) const;
+    TravelResult plan(DimensionId from, DimensionId to, const Vec3i& pos,
+                      const std::vector<Vec3i>& knownPortals) const;
+
+    bool findSafeSpot(DimensionId to, const Vec3i& near, int radius, int clearance,
+                      Vec3i& outSpot) const;
+    void carvePlatform(DimensionId to, const Vec3i& spot, int halfWidth, int clearance);
+
+private:
+    DimensionSet& worlds;
+};
+
+} // namespace vf